test_runner: fix lazy test.assert accessor

[Renegade334]

The intention was for this to be a define-on-access lazy property, but the object passed to Object.defineProperty isn't a descriptor, so this never happens. As a result, the assert object is currently reconstructed on every access; test.assert === test.assert is false at present.

Changed to a conventional lazy getter, in alignment with the other node:test property accessors.

[nodejs-github-bot]

Review requested:

• @nodejs/test_runner

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.53%. Comparing base (4f24aff) to head (deb3808).
⚠️ Report is 21 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #61097   +/-   ##
=======================================
  Coverage   88.53%   88.53%           
=======================================
  Files         703      703           
  Lines      208546   208550    +4     
  Branches    40217    40212    -5     
=======================================
+ Hits       184634   184638    +4     
+ Misses      15926    15921    -5     
- Partials     7986     7991    +5     

Files with missing lines	Coverage Δ
lib/test.js	100.00% <100.00%> (ø)

... and 24 files with indirect coverage changes

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[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/70510/

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/70546/

[nodejs-github-bot]

Landed in 3b7477c