test: update CMake minimum version

[Old-Ding]

Summary

• raise the test CMake minimum version from 2.8 to 3.14
• align the declared minimum with the existing FetchContent_MakeAvailable() usage
• skip the Codecov upload step when CODECOV_TOKEN is unavailable, so fork PRs are not failed by tokenless upload rate limits after tests pass

Testing

• git diff --check
• git ls-files --eol test\CMakeLists.txt .github\workflows\unit-tests.yml
• static source check that the CMake file declares 3.14 and the workflow only runs Codecov with a token
• GitHub Actions before the CI fix built and ran the unit test binary successfully: All tests passed (972 assertions in 306 test cases); the job failed only in the Codecov upload step with a 429 tokenless upload rate limit

Not run locally: unit tests. This Windows environment does not have CMake or a C/C++ compiler available in PATH.

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