test_runner: fix watch cwd with isolation none

[trivikr]

When run({ watch: true, cwd, isolation: 'none' }) spawned the watch child,
it reused the parent process argv while changing the child process cwd. That
could make the child run the outer test file, recurse, or miss the test file
that was discovered under the configured cwd.

This updates the isolation-none watch path to spawn the child test runner with
the current run's explicit files or glob patterns instead of process.argv.
A regression test verifies that the cwd-scoped test file actually runs.

Fixes: #63689

---

Fixes a flaky failure in test-runner/test-run-watch-cwd-isolation-none.
Refs: https://github.com/nodejs/reliability/blob/main/reports/2026-06-01.md#jstest-failure

Example error log

not ok 5115 test-runner/test-run-watch-cwd-isolation-none
  ---
  duration_ms: 1588.88400
  severity: fail
  exitcode: 1
  stack: |-
  ...

---

Assisted-by: openai:gpt-5.5

[nodejs-github-bot]

Review requested:

• @nodejs/test_runner

[codecov]

Codecov Report

❌ Patch coverage is 83.33333% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 90.35%. Comparing base (aa444b2) to head (423a1e2).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/internal/test_runner/runner.js	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #63690      +/-   ##
==========================================
- Coverage   90.35%   90.35%   -0.01%     
==========================================
  Files         732      732              
  Lines      236527   236531       +4     
  Branches    44535    44536       +1     
==========================================
- Hits       213717   213716       -1     
+ Misses      14533    14531       -2     
- Partials     8277     8284       +7     

Files with missing lines	Coverage Δ
lib/internal/test_runner/runner.js	94.40% <83.33%> (-0.07%)	⬇️

... and 22 files with indirect coverage changes

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[Han5991]

Could this test program live as a fixture under test/fixtures/test-runner-watch/ for consistency with the other watch/cwd tests, instead of the inline template string?

[trivikr]

The other files in the fixtures directory are pure JS files, which are read as-is.
The string in this template contains a variable which prints the marker string. So it can't have .js prefix.

I'll keep it as-is unless there's a strong recommendation to move it to fixtures folder.