Add tests for mapping behavior around device.destroy()

[kainino0x]

Test mappedAtCreation and mapAsync behavior around device.destroy(). Searching for references to GPUDevice.destroy, I can't seem to find any other tests for this, except for map_detach which I've also enhanced a tiny bit here.
Spec: https://gpuweb.github.io/gpuweb/#dom-gpubuffer-mapasync

This tests the behavior currently written in the spec (modulo uncertainty around gpuweb/gpuweb#5101), but the specced behavior is weird; see gpuweb/gpuweb#5102.

Also fix a bug where some test code was running twice unintentionally.

Issue: None filed except for the spec 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 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.

[kainino0x]

Also fix a bug where some test code was running twice unintentionally.

fn is already called below, it should not be called twice. We don't need to capture validation errors anymore because there are never any (and test teardown will check this). (Likely when this test was written, sometimes there were validation errors here.)

[kainino0x]

I'll revisit this after the WG discusses gpuweb/gpuweb#5102.

[lokokung]

We are still pending this decision right?

[kainino0x]

Yes.