test: fix flaky test-runner coverage threshold test

[trivikr]

This fixes a flaky assertion in parallel/test-runner-run-coverage.mjs.

The threshold subtest was asserting that process.exitCode was not already
1 before starting its coverage run. That value is process-global, so it
can be affected by other run() coverage activity in the same process.

The test now clears process.exitCode before the threshold check and
restores the original value with a cleanup hook.

Refs: https://github.com/nodejs/reliability/issues?q=sort%3Aupdated-desc%20is%3Aissue%20state%3Aopen%20%22test-runner-run-coverage%22

Example

not ok 3959 parallel/test-runner-run-coverage
  ---
  duration_ms: 710.47400
  severity: fail
  exitcode: 1
  stack: |-
    Test failure: 'should run with coverage and fail when below line threshold'
    Location: test/parallel/test-runner-run-coverage.mjs:157:11
    AssertionError [ERR_ASSERTION]: Expected "actual" to be strictly unequal to: 1
        at TestContext.<anonymous> (file:///home/iojs/build/workspace/node-test-commit-linux-containered/test/parallel/test-runner-run-coverage.mjs:160:14)
        at Test.runInAsyncScope (node:async_hooks:226:14)
        at Test.run (node:internal/test_runner/test:1382:25)
        at Suite.processPendingSubtests (node:internal/test_runner/test:960:18)
        at Test.postRun (node:internal/test_runner/test:1522:19)
        at Test.run (node:internal/test_runner/test:1447:12)
        at process.processTicksAndRejections (node:internal/process/task_queues:104:5)
        at async Suite.processPendingSubtests (node:internal/test_runner/test:960:7) {
      generatedMessage: true,
      code: 'ERR_ASSERTION',
      actual: 1,
      expected: 1,
      operator: 'notStrictEqual',
      diff: 'simple'
    }

[nodejs-github-bot]

Review requested:

• @nodejs/test_runner

[atlowChemi]

@trivikr I think the commit should be under the test: submodule

[trivikr]

I think the commit should be under the test: submodule

Yes. Since the code changes are only in test files.
I've updated the commit message and requested a review.

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/74354/

[github-actions]

Fast-track has been requested by @trivikr. Please 👍 to approve.