test: avoid test_runner watch restart in spec snapshot

[trivikr]

This updates the test_runner watch-mode spec reporter fixture to pass
explicit test files to run().

The fixture no longer watches the whole temporary directory, so setup writes
cannot race with watcher initialization and emit an unexpected restart banner.

Refs:

• https://github.com/nodejs/node/actions/runs/25983032903/job/76375266688
• https://github.com/nodejs/reliability/blob/main/reports/2026-05-12.md#jstest-failure

---

Assisted-by: openai:gpt-5.5

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.06%. Comparing base (bc90667) to head (d501a41).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #63392   +/-   ##
=======================================
  Coverage   90.05%   90.06%           
=======================================
  Files         714      714           
  Lines      225656   225656           
  Branches    42696    42701    +5     
=======================================
+ Hits       203214   203227   +13     
+ Misses      14220    14216    -4     
+ Partials     8222     8213    -9     

see 30 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/73536/