Move builtin must_use tests into per-builtin validation suites

[jrprice]

We have tests that the results of most builtins must be used in must_use.spec.ts, but we have also started adding equivalent tests inside the per-builtin validation suites, e.g. #3496.

We should probably move all of these must_use tests into the per-builtin suites to make it clearer what should be covered when adding a new builtin. The dedicated must_use suite would then focus on user-specified @must_use attributes.

[alan-baker]

Don't neglect src/webgpu/shader/validation/expression/call/builtin/value_constructor.spec.ts.