Add tests for subgroupShuffle variants

[alan-baker]

• Refactor some duplicated code into subgroup_util.ts
  • typed input generation
  • framebuffer parameter generation
• Add tests for subgroup shuffle variants
  • All variants: subgroupShuffle, subgroupShuffleUp, subgroupShuffleDown, subgroupShuffleXor
  • Fragment tests
  • Data type tests
  • specific variant tests
  • randomized compute tests
  • predicated compute tests

Issue: #

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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 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.

[dneto0]

This is a lot and I may have missed things.
LGTM.