test_runner: display diagnostic messages in dot reporter

[armorbreak001]

Problem

When running tests with both the dot reporter (--test-reporter dot) and coverage enabled, if coverage setup or collection fails, the process exits with a non-zero status code but shows no output explaining why. The user only sees test dots and then an unexplained failure.

Proposed Fix

Collect test:diagnostic events in the dot reporter and display them after the test results (similar to how failed tests are shown). This ensures coverage warnings and other diagnostic messages are visible.

Changes

lib/internal/test_runner/reporter/dot.js:

• Added diagnostics[] array to collect diagnostic events
• Handle test:diagnostic type in the event loop
• Display diagnostics section after failed tests section (yellow ! prefix)

Before

.....X....X......
Failed tests:
  ...
$ echo $?
1
# No indication of WHY it failed if coverage was the issue

After

.....X....X......
Failed tests:
  ...

Diagnostics:

! Warning: Code coverage could not be enabled. ...
! Warning: Could not report code coverage. ...
$ echo $?
1

Approach

• Follows the same pattern as existing failed tests collection/display
• Uses yellow color to differentiate from red failures
• Minimal change — only adds diagnostic event handling
• Consistent with how spec reporter already handles diagnostics

Fixes: #60884

[nodejs-github-bot]

Review requested:

• @nodejs/test_runner