Add unary, function, and uniformity tests for WGSL immediate pointers (#4677)

* Add unary expression tests for WGSL immediate pointers

This patch extends address-of and indirection validation to
`var<immediate>`.

The new cases verify that shaders can take the address of immediate
variables, read through `ptr<immediate>` values, and reject writes
through those pointers. The generated matrix is limited to store types
that are valid for `immediate`; invalid store types are covered by the
declaration and pointer type suites.

* Add function tests for WGSL immediate pointers

This patch extends function restriction tests for
`ptr<immediate, u32>` parameters.

The new cases verify that immediate pointer parameters follow the
`unrestricted_pointer_parameters` language feature gate, that calls can
pass the address of a `var<immediate>` to a matching parameter, and
that explicit read and write access modes on immediate pointer
parameters are rejected.

* Add uniformity tests for WGSL immediate reads

This patch adds `var<immediate>` reads as a uniform condition source
in the uniformity suite.

The coverage verifies that control flow depending on immediate data is
treated as uniform in the baseline uniformity cases and in the subgroup
variants. The cases are skipped unless immediate data support is
exposed by the browser.