Run batchFinishedCallback on processBatch to catch unhandled rejections by test functions

[sagudev]

Issue: #3809 (I verified that it fixes the timeout issue in servo)

Related #3158, but I think this fix is more appropriate (and less invasive).

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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 in the test tree.
• Test descriptions allow a reader to "read only the test plans and evaluate coverage completeness", and accurately reflect the test code.
• 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.

[sagudev]

Live version: https://sagudev.github.io/cts/standalone/?runnow=1&q=webgpu:shader,execution,expression,binary,f32_subtraction:scalar_vector:inputSource=%22const%22;* with pipeline reject hardcoded.

[sagudev]

Care to review @kainino0x?

[kainino0x]

Sorry, had this in my review inbox but been swamped this week!