diff --git a/tests/phpunit/tests/fonts/font-face/base.php b/tests/phpunit/tests/fonts/font-face/base.php
index 3b016557728a6..43c21ac3a89b3 100644
--- a/tests/phpunit/tests/fonts/font-face/base.php
+++ b/tests/phpunit/tests/fonts/font-face/base.php
@@ -69,6 +69,7 @@ public static function set_up_before_class() {
 	public static function tear_down_after_class() {
 		// Reset static flags.
 		self::$requires_switch_theme_fixtures = false;
+		self::$theme_root                     = null;
 
 		parent::tear_down_after_class();
 	}
diff --git a/tests/phpunit/tests/fonts/font-face/wpPrintFontFacesFromStyleVariations.php b/tests/phpunit/tests/fonts/font-face/wpPrintFontFacesFromStyleVariations.php
index 664404d552b45..2c35569b4d196 100644
--- a/tests/phpunit/tests/fonts/font-face/wpPrintFontFacesFromStyleVariations.php
+++ b/tests/phpunit/tests/fonts/font-face/wpPrintFontFacesFromStyleVariations.php
@@ -16,8 +16,9 @@ class Tests_Fonts_WpPrintFontFacesFromStyleVariations extends WP_Font_Face_UnitT
 	const FONTS_THEME = 'fonts-block-theme';
 
 	public static function set_up_before_class() {
-		parent::set_up_before_class();
 		self::$requires_switch_theme_fixtures = true;
+
+		parent::set_up_before_class();
 	}
 
 	/**
diff --git a/tests/phpunit/tests/rest-api/rest-block-type-controller.php b/tests/phpunit/tests/rest-api/rest-block-type-controller.php
index 3cf8c5244d77c..21eb081bcf11e 100644
--- a/tests/phpunit/tests/rest-api/rest-block-type-controller.php
+++ b/tests/phpunit/tests/rest-api/rest-block-type-controller.php
@@ -62,8 +62,6 @@ public static function wpTearDownAfterClass() {
 		self::delete_user( self::$admin_id );
 		self::delete_user( self::$subscriber_id );
 		unregister_block_type( 'fake/test' );
-		unregister_block_type( 'fake/invalid' );
-		unregister_block_type( 'fake/false' );
 	}
 
 	/**
@@ -138,7 +136,8 @@ public function test_get_item_with_styles() {
 		wp_set_current_user( self::$admin_id );
 		$request  = new WP_REST_Request( 'GET', '/wp/v2/block-types/' . $block_name );
 		$response = rest_get_server()->dispatch( $request );
-		$data     = $response->get_data();
+		unregister_block_type( $block_name );
+		$data = $response->get_data();
 		$this->assertSameSets( array( $block_styles ), $data['styles'] );
 	}
 
@@ -166,6 +165,7 @@ public function test_get_item_with_styles_merge() {
 		wp_set_current_user( self::$admin_id );
 		$request  = new WP_REST_Request( 'GET', '/wp/v2/block-types/' . $block_name );
 		$response = rest_get_server()->dispatch( $request );
+		unregister_block_type( $block_name );
 		$data     = $response->get_data();
 		$expected = array(
 			array(
@@ -232,7 +232,8 @@ public function test_get_item_invalid() {
 		wp_set_current_user( self::$admin_id );
 		$request  = new WP_REST_Request( 'GET', '/wp/v2/block-types/' . $block_type );
 		$response = rest_get_server()->dispatch( $request );
-		$data     = $response->get_data();
+		unregister_block_type( $block_type );
+		$data = $response->get_data();
 		$this->assertSame( $block_type, $data['name'] );
 		$this->assertSame( '1', $data['title'] );
 		$this->assertNull( $data['category'] );
@@ -311,7 +312,8 @@ public function test_get_item_defaults() {
 		wp_set_current_user( self::$admin_id );
 		$request  = new WP_REST_Request( 'GET', '/wp/v2/block-types/' . $block_type );
 		$response = rest_get_server()->dispatch( $request );
-		$data     = $response->get_data();
+		unregister_block_type( $block_type );
+		$data = $response->get_data();
 		$this->assertSame( $block_type, $data['name'] );
 		$this->assertSame( '', $data['title'] );
 		$this->assertNull( $data['category'] );
@@ -368,7 +370,8 @@ public function test_get_item_deprecated() {
 		wp_set_current_user( self::$admin_id );
 		$request  = new WP_REST_Request( 'GET', '/wp/v2/block-types/' . $block_type );
 		$response = rest_get_server()->dispatch( $request );
-		$data     = $response->get_data();
+		unregister_block_type( $block_type );
+		$data = $response->get_data();
 		$this->assertSameSets(
 			array( 'hello_world' ),
 			$data['editor_script_handles'],
@@ -438,7 +441,8 @@ public function test_get_item_deprecated_with_arrays() {
 		wp_set_current_user( self::$admin_id );
 		$request  = new WP_REST_Request( 'GET', '/wp/v2/block-types/' . $block_type );
 		$response = rest_get_server()->dispatch( $request );
-		$data     = $response->get_data();
+		unregister_block_type( $block_type );
+		$data = $response->get_data();
 		$this->assertSameSets(
 			$settings['editor_script'],
 			$data['editor_script_handles'],
@@ -525,7 +529,8 @@ public function test_get_variation() {
 		wp_set_current_user( self::$admin_id );
 		$request  = new WP_REST_Request( 'GET', '/wp/v2/block-types/' . $block_type );
 		$response = rest_get_server()->dispatch( $request );
-		$data     = $response->get_data();
+		unregister_block_type( $block_type );
+		$data = $response->get_data();
 		$this->assertSame( $block_type, $data['name'] );
 		$this->assertArrayHasKey( 'variations', $data );
 		$this->assertCount( 1, $data['variations'] );
@@ -868,7 +873,8 @@ public function test_variation_callback() {
 		wp_set_current_user( self::$admin_id );
 		$request  = new WP_REST_Request( 'GET', '/wp/v2/block-types/' . $block_type );
 		$response = rest_get_server()->dispatch( $request );
-		$data     = $response->get_data();
+		unregister_block_type( $block_type );
+		$data = $response->get_data();
 		$this->assertSameSets( $this->mock_variation_callback(), $data['variations'] );
 	}
 
diff --git a/tests/phpunit/tests/sitemaps/sitemaps.php b/tests/phpunit/tests/sitemaps/sitemaps.php
index 85f9965245842..24243787d5068 100644
--- a/tests/phpunit/tests/sitemaps/sitemaps.php
+++ b/tests/phpunit/tests/sitemaps/sitemaps.php
@@ -464,15 +464,17 @@ public function test_sitemaps_enabled() {
 
 	/**
 	 * @ticket 50643
-	 * @runInSeparateProcess
-	 * @preserveGlobalState disabled
 	 */
 	public function test_disable_sitemap_should_return_404() {
 		add_filter( 'wp_sitemaps_enabled', '__return_false' );
 
+		// Instantiate the server before navigating: registering the sitemap
+		// rewrite tags is what adds `sitemap` to `$wp->public_query_vars`.
+		$sitemaps = wp_sitemaps_get_server();
+
 		$this->go_to( home_url( '/?sitemap=index' ) );
 
-		wp_sitemaps_get_server()->render_sitemaps();
+		$sitemaps->render_sitemaps();
 
 		remove_filter( 'wp_sitemaps_enabled', '__return_false' );
 
@@ -481,8 +483,6 @@ public function test_disable_sitemap_should_return_404() {
 
 	/**
 	 * @ticket 50643
-	 * @runInSeparateProcess
-	 * @preserveGlobalState disabled
 	 */
 	public function test_empty_url_list_should_return_404() {
 		wp_register_sitemap_provider( 'foo', new WP_Sitemaps_Empty_Test_Provider( 'foo' ) );
diff --git a/tests/phpunit/tests/widgets/wpWidgetMediaAudio.php b/tests/phpunit/tests/widgets/wpWidgetMediaAudio.php
index 20aee5f30baf4..cb555fb5285a5 100644
--- a/tests/phpunit/tests/widgets/wpWidgetMediaAudio.php
+++ b/tests/phpunit/tests/widgets/wpWidgetMediaAudio.php
@@ -316,6 +316,9 @@ public function test_enqueue_admin_scripts() {
 	 * @covers WP_Widget_Media_Audio::render_control_template_scripts
 	 */
 	public function test_render_control_template_scripts() {
+		// Provides wp_underscore_audio_template(), normally loaded by wp_enqueue_media().
+		require_once ABSPATH . WPINC . '/media-template.php';
+
 		$widget = new WP_Widget_Media_Audio();
 
 		ob_start();
diff --git a/tests/phpunit/tests/widgets/wpWidgetMediaVideo.php b/tests/phpunit/tests/widgets/wpWidgetMediaVideo.php
index aeedae2a3174f..98c86ec4865fd 100644
--- a/tests/phpunit/tests/widgets/wpWidgetMediaVideo.php
+++ b/tests/phpunit/tests/widgets/wpWidgetMediaVideo.php
@@ -344,6 +344,9 @@ public function test_enqueue_admin_scripts() {
 	 * @covers WP_Widget_Media_Video::render_control_template_scripts
 	 */
 	public function test_render_control_template_scripts() {
+		// Provides wp_underscore_video_template(), normally loaded by wp_enqueue_media().
+		require_once ABSPATH . WPINC . '/media-template.php';
+
 		$widget = new WP_Widget_Media_Video();
 
 		ob_start();