attemptGarbageCollection periodically (every N subcases)

[kainino0x]

Runs attemptGarbageCollection() after every N subcases (5000 by default)

The subcase counting is global so this introduces a little variance from run to run. Alternative would be: Attempt GC at the end of every case (this seems too often), plus attempt GC every N subcases within a case so it's more deterministic. But it's still not totally deterministic because the subcases are running concurrently.

For webgpu:shader,validation,expression,call,builtin,faceForward:* (26336 subcases) in Chrome (very rough because the numbers were not coming out very clean):

N	without flag	with --js-flags=--expose-gc
Infinity	2.9s	3.1s (no idea why this is slower)
5000	roughly same	roughly same
1000	3.1s (7%)	3.25s (5%)
100	3.4s (17%)	4.1s (37%)

Issue: Next attempt at #4006 to fix OOMs seen in https://crbug.com/373485785

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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.)
• [not tested] Test have be tested with compatibility mode validation enabled and behave as expected. (If not passing, explain above.)

Requirements for reviewer sign-off:

• [n/a] Tests are properly located in the test tree.
• [n/a] Test descriptions allow a reader to "read only the test plans and evaluate coverage completeness", and accurately reflect the test code.
• [n/a] Tests provide complete coverage (including validation control cases). Missing coverage MUST be covered by TODOs.
• Helpers and types promote readability and maintainability.

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