Add insertBits edge case regression test

[sudonatalie]

This reproduces a GPU hang on Mac for the Dawn implementation (see crbug.com/506180954).

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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:

• 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.
• 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 aee4691):

+webgpu:shader,execution,expression,call,builtin,insertBits:array_index:* - 1 cases, 1 subcases (~1/case)
-TOTAL: 281385 cases, 2323156 subcases
+TOTAL: 281386 cases, 2323157 subcases

[sudonatalie]

Currently fails on my M4 Mac and passes following: https://dawn-review.googlesource.com/c/dawn/+/316515