Add tests for `texture_formats_tier` WGSL language feature

[andyleiserson]

The WGSL language feature was recently added: gpuweb/gpuweb#5524

As discussed in #4567 (comment), two things should be done to add test coverage for this extension:

1. A new test in shader,validation,extension for the texture_formats_tier1 extension. This test should verify that using the requires directive or one of the 23 new format identifiers is allowed if-and-only-if the WGSL feature is available.
2. Remove the consideration in shader,validation,extension,readonly_and_readwrite_storage_textures of the device feature. This was added temporarily so the test could still pass on systems that implement texture_format_tier1 but do not yet report support for the new language feature.