Improve 'args' and 'must_use' validation tests for several builtins

[dneto0]

Responds to review feedback on sin, cos tests.

Updates:
dot4IPacked
dot4UPacked
floor
pack4xI8
pack4xI8Clamp
pack4xU8
pack4xU8Clamp
unpack4xI8
unpack4xU8

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.

[ben-clayton]

I still find the numbers in the field names confusing, but otherwise LGTM!