Speed up shader,execution,expression tests with async pipelines

[toji]

Some of these tests are among the longest running that we have, so hopefully this has a dramatic impact on runtimes. Profiling shows that almost all the time is being spent waiting for pipeline creation. By using createComputePipelineAsync, however, we ensure that multiple shaders in a batch can be compiling at once, which drops test run times significantly.

For example, the webgpu:shader,execution,expression,binary,af_matrix_subtraction:matrix:inputSource="const";cols=4;rows=4 test, one of the longest running in the entire CTS, was taking 5 minutes to run on my Windows machine. With this patch it completes in 46 seconds 80 seconds on the same device.

In general I'm seeing speedups of 5x-6x (sorry, it's only 4x. See comments below) on the tests that I've tried.

To demonstrate why, we can look at this chunk of a trace from the shorter webgpu:shader,execution,expression,binary,af_matrix_subtraction:matrix:inputSource="const";cols=2;rows=2 test, which was previously taking 8 seconds to run.

The blue chunks are all pipeline compiles, and you can see they dominate the runtime and are being performed serially. With this patch it completes in about 2 seconds and the tracing produces results like this:

Same amount of blue, but now more parallel!

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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.)

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.

[ben-clayton]

NICE. LGTM, but I think Austin should approve.

[austinEng]

nice!