test: clean up the near-heap-limit snapshot test's child and temp dir on failure - #54940

Description of Change

utilityProcess module behavior supports generating snapshots via v8.setHeapSnapshotNearHeapLimit has been running close to its 60 s budget on upstream CI. Passing macos-x64 runs take up to 56 s, and main run 38585284171 needed a retry on linux-x64-asan (98.6 s) and macos-x64 (94.3 s). #54901 sped up the oom-grow.js fixture that made it slow.

This PR fixes what happens when an attempt still fails or times out:

• Nothing killed the attempt's utility process, so it kept running during CI's retry.
• The diagnostic directory was removed only when the test passed.

The test now registers both cleanups with defer() as soon as they are needed: deferKillUtilityProcess(child) right after fork(), and a forced fs.rm() of the directory right after mkdtemp(). Cleanup runs in reverse order, so the child is killed before its directory is removed.

Tested with a fixture that stalls below the heap limit and retry: 1:

• Before this change, two stalled children were alive at once and two directories were left in the temp dir.
• After it, at most one child was alive and no directories were left.

The automatic v8.setHeapSnapshotNearHeapLimit(1) path and the exactly-one, non-empty snapshot assertions are unchanged.

On 42-x-y and 43-x-y, the backport also needs defer added to the ./lib/spec-helpers.ts import; otherwise the test throws ReferenceError: defer is not defined.

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Checklist

• I have built and tested this change
• I have filled out the PR description
• I have reviewed and verified the changes
• tests are changed or added

Release Notes

Notes: none