Allow tests to extend endTestScope timeout

[haoxli]

Added a per-test endTestScope timeout function so heavy cases can extend the time without slowing down the rest of the suite, and used it to give the 64K timestamp-query stress test run extra headroom, avoiding flakes while leaving the default 5000ms limit untouched elsewhere.

Issue: #

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Requirements for PR author:

• All missing test coverage is tracked with "TODO" or .unimplemented().
• New helpers are /** documented */ and new helper files are found in helper_index.txt.
• Test behaves as expected in a WebGPU implementation. (If not passing, explain above.)
• Test have be tested with compatibility mode validation enabled and behave as expected. (If not passing, explain above.)

Requirements for reviewer sign-off:

• Tests are properly located.
• Test descriptions are accurate and complete.
• Tests provide complete coverage (including validation control cases). Missing coverage MUST be covered by TODOs.
• Tests avoid over-parameterization (see case count report).

When landing this PR, be sure to make any necessary issue status updates.

[github-actions]

Results for build job (at 52edcaa):

 Test case/subcase counts did not change.

[kainino0x]

I'm not sure if I want to make this part of the "official" interface of the test fixture. It seems a bit fragile or easy to misuse because it's very specific to the test why this might need to be done.

Would it work to wait inside the test by just doing await t.queue.onSubmittedWorkDone() at the end before returning from the test function?