fix(dx12): Map components properly for textureLoad from stencil

[andyleiserson]

The shader view format used by the dx12 backend for stencil-only views, DXGI_FORMAT_X24_TYPELESS_G8_UINT, places the value in the green component. WebGPU requires it in the red component.

Fixes #6620 using the approach suggested by the reporter: configure a D3D12_SHADER_COMPONENT_MAPPING swizzle in the view.

Testing
Enables CTS tests (it's not clear that this coverage is the primary goal of these CTS tests, but I couldn't find anything better, and coverage is coverage). This change fixes the two format="stencil8" tests; it also adds other tests that were already passing to test.lst, and other pre-existing failures to fail.lst.

Squash or Rebase? Squash

Checklist

• I self-reviewed and fully understand this PR.
• WebGPU implementations built with wgpu may be affected behaviorally.
• Validation and feature gates are in place to confine behavioral changes.
• Tests demonstrate the validation and altered logic works.
• CHANGELOG.md entries for the user-facing effects of this change are present.
• The PR is minimal, and doesn't make sense to land as multiple PRs.
• Commits are logically scoped and individually reviewable.
• The PR description has enough context to understand the motivation and solution implemented.