Limits test: skip duplicate cases when maximum limit == default limit

[kainino0x]

Also update some tangentially-related TODOs.

Tested by trying webgpu:api,validation,capability_checks,limits,maxBindGroups:setBindGroup,at_over:* in Chrome which doesn't expose a higher value for maxBindGroups.

Issue: part of #4561

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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:

• [n/a] Tests are properly located.
• Test descriptions are accurate and complete.
• Tests provide complete coverage (including validation control cases). Missing coverage MUST be covered by TODOs.
• [n/a] Tests avoid over-parameterization (see case count report).

When landing this PR, be sure to make any necessary issue status updates.

[github-actions]

Results for build job (at 0713ad4):

 Test case/subcase counts did not change.