test_runner: filter execArgv fallback for child tests

[trivikr]

Fixes flaky failures in parallel/test-runner-flag-propagation.

Apply the test runner propagation filter to process.execArgv entries that
are not reported by the options binding.

Some flags, such as config-file/test-runner flags, can reach the isolated
child test fallback path through process.execArgv. Without filtering that
path, flags like --experimental-config-file may leak into child tests and
re-enable options that should not propagate, including
--experimental-test-coverage.

This change keeps preserving V8-only flags such as --allow-natives-syntax,
while filtering test-runner control flags consistently across both
option-binding flags and fallback execArgv entries.

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

Example

not ok 3946 parallel/test-runner-flag-propagation
  ---
  duration_ms: 6171.58100
  severity: fail
  exitcode: 1
  stack: |-
    Test failure: 'should propagate --experimental-test-coverage from config file (test) to child tests'
    Location: test/parallel/test-runner-flag-propagation.js:94:7
    AssertionError [ERR_ASSERTION]: Config file propagation test failed for --experimental-test-coverage.
    
    1 !== 0
    
        at TestContext.<anonymous> (/home/iojs/build/workspace/node-test-commit-linux-containered/test/parallel/test-runner-flag-propagation.js:126:16)
        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 async Suite.processPendingSubtests (node:internal/test_runner/test:960:7) {
      generatedMessage: false,
      code: 'ERR_ASSERTION',
      actual: 1,
      expected: 0,
      operator: 'strictEqual',
      diff: 'simple'
    }

---

Assisted-by: openai:gpt-5.5

[nodejs-github-bot]

Review requested:

• @nodejs/test_runner

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/74348/