test_runner: automatically wait for subtests to finish

[LiviaMedeiros]

This commit updates the test runner to automatically wait for subtests to finish. This makes the experience more consistent with suites and removes the need to await anything.

Extracted from #56664 (aa3523e) which was reverted in #58282 due to breakage caused by other parts of it.

Authorship belongs to @cjihrig.

To minimize problems in userland, it would be highly preferable to ship this in the same release as #58282 (7a0c74b in particular), since aa3523e was already shipped in v24.0.0 and releasing this PR would keep everything as-is for current v24.x users.
This should not land on previous major release lines because #56664 was semver-majorPRs that contain breaking changes and should be released in the next major version. (although IMHO this particular change is backportable)

Cherry-pick was not 100% clean, there was a trivial conflict with #56765 at the end of lib/internal/test_runner/test.js diff, cc @pmarchini just in case.

cc @cjihrig@romainmenke @nodejs/test_runner

[nodejs-github-bot]

Review requested:

• @nodejs/test_runner

[codecov]

Codecov Report

Attention: Patch coverage is 96.29630% with 1 line in your changes missing coverage. Please review.

Project coverage is 90.08%. Comparing base (04cb572) to head (0767b79).
Report is 11 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/internal/test_runner/harness.js	92.85%	1 Missing ⚠️

Additional details and impacted files

@@ Coverage Diff @@## main #58800 +/- ##
=======================================
Coverage 90.07% 90.08% =======================================
Files 640 640 Lines 188262 188286 +24 Branches 36912 36915 +3 =======================================
+ Hits 169580 169612 +32 + Misses 11399 11396 -3 + Partials 7283 7278 -5 

Files with missing lines	Coverage Δ
lib/internal/test_runner/test.js	97.34% <100.00%> (+0.02%)	⬆️
lib/internal/test_runner/harness.js	93.33% <92.85%> (+0.19%)	⬆️

... and 37 files with indirect coverage changes

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[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/67624/ 💛

[RafaelGSS]

Landed in dff081e