test_runner: make skip/todo/expectFailure use JS truthiness

[VaishnavIUpadyaya]

Fixes #61815

Updates skip, todo, and expectFailure handling in the test runner
to use JavaScript truthiness instead of explicit undefined/false checks.

This ensures:

• Empty string ('') is treated as false
• Behavior matches the documented "truthy" semantics

Adds a test to verify that skip: '' does not skip the test.

[nodejs-github-bot]

Review requested:

• @nodejs/test_runner

[JakobJingleheimer]

Looks like the doc already state truthiness too.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.68%. Comparing base (1989f4d) to head (d10c419).
⚠️ Report is 40 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #62346      +/-   ##
==========================================
+ Coverage   89.65%   89.68%   +0.02%     
==========================================
  Files         676      676              
  Lines      206555   206694     +139     
  Branches    39547    39577      +30     
==========================================
+ Hits       185195   185377     +182     
+ Misses      13495    13446      -49     
- Partials     7865     7871       +6     

Files with missing lines	Coverage Δ
lib/internal/test_runner/test.js	96.80% <100.00%> (+<0.01%)	⬆️

... and 47 files with indirect coverage changes

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