Check GPUAdapterInfo.isFallbackAdapter on explicit fallback request

[tharkum]

According to specification a fallback adapter may still be returned if forceFallbackAdapter is set to false and either no other appropriate adapter is available or the user agent chooses to return a fallback adapter.
https://gpuweb.github.io/gpuweb/#dom-gpurequestadapteroptions-forcefallbackadapter

Therefore, checking whether an adapter is a fallback adapter should only be performed if forceFallbackAdapter is true.

Issue: fixes #4480

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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.

[tharkum]

@kainino0x < Please take a look.

[kainino0x]

Sorry, got behind on reviews for a few days. Thank you for the fix!