With this WebGPU spec change gpuweb/gpuweb#3348 buffer.mapAsync() rejects immediately if it is pending map. This commit adds a test to check it.

The test is based on https://bugs.chromium.org/p/chromium/issues/detail?id=1355994#c3 that @kainino0x suggested.

Issue: #2009

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.)

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.