test: capture negative utimes mtime at call time

[inoway46]

test-fs-utimes recalculates the expected mtime during assertion.

For negative numeric timestamps such as -1, Node normalizes the value to the current time at call time. Recomputing the expectation later can let it drift past the test's 2-second tolerance and cause a false failure.

This change captures the expected mtime immediately before each utimes/lutimes/futimes call and reuses it for the assertion.

Background: #62490 (comment)

Refs: #37692

[inoway46]

I noticed this in my current PR. #62471 (comment)

The same failure has shown up twice recently on node-test-commit-arm (ubuntu2404-arm64):

• https://ci.nodejs.org/job/node-test-commit-arm/nodes=ubuntu2404-arm64/62676/testReport/junit/(root)/parallel/test_fs_utimes/
• https://ci.nodejs.org/job/node-test-commit-arm/nodes=ubuntu2404-arm64/62686/testReport/junit/(root)/parallel/test_fs_utimes/

[inoway46]

This helper is optional and I can inline it if preferred.

I kept it as a helper because the key point is to capture the expected mtime at call time before each invocation, and I thought naming that step made the intent clearer.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.71%. Comparing base (cc96741) to head (5246b06).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #62490      +/-   ##
==========================================
- Coverage   91.55%   89.71%   -1.84%     
==========================================
  Files         351      692     +341     
  Lines      147653   213986   +66333     
  Branches    23224    41052   +17828     
==========================================
+ Hits       135179   191982   +56803     
- Misses      12217    14086    +1869     
- Partials      257     7918    +7661     

see 462 files with indirect coverage changes

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