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 41ddee9):

 Test case/subcase counts did not change.

[kainino0x]

Fixed some nits, LGTM

• try should be around the whole test just in case anything throws an exception (even though it shouldn't, and IIRC the device will get discarded afterward anyway if that happens)
• comments to explain what's going on