Test maxBindGroupsPlusVertexBuffers limits

[greggman]

Here's an updated test for maxBindGroupPlusVertexBuffers. I can't easily test with explicit pipeline layouts because it's currently impossible to create sparse explicit pipeline layouts but the code is there for when it becomes possible which seems like a future plan.

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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.

[greggman]

Note: All of the tests are skipped on Chrome since Chrome doesn't currently set maxBindGroupsPlusVertexBuffers high enough to be able to test anything.

On Safari Technology Preview the tests that are within the limits pass, those that are past the limit fail as not generating an error. For the draw tests, they current fail because Safari disallows calling setVertexBuffer(ndx, null). If you comment out the sections that do that then, as above, they pass when things are within the limit and fail with "no error when one was expected" when over the limit.

[mwyrzykowski]

On Safari Technology Preview the tests that are within the limits pass, those that are past the limit fail as not generating an error. For the draw tests, they current fail because Safari disallows calling setVertexBuffer(ndx, null). If you comment out the sections that do that then, as above, they pass when things are within the limit and fail with "no error when one was expected" when over the limit.

I'll try this locally since Safari Technology Preview doesn't include validation for these tests yet, I just noticed it while adding it locally. I have a PR open to add the validation, it hasn't merged yet.

[greggman]

I updated to use empty bindGroupLayouts as suggested by @kainino0x so the explicit pipeline layouts path is now enabled.

[kainino0x]

LGTM