Fix maxBindGroupsPlusVertexBuffers test

[greggman]

I think this works now. I tested by hacking up the CTS to force a lower limit that Chrome could test

see: https://gist.github.com/greggman/c3c56b8d53174453df166a5a2383b1c3#maxbindgroupsplusvertexbuffers

I also changed it so it can test at limit tests (vs over limit tests). This should mean some tests run on browsers where maxBindGroups + maxVertexBuffers === maxBindGroupsPlusVertexBuffers.

Hopefully it's correct now.

Issue: #3376

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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.