Expand textureStorage tests

[greggman]

• test 2d, 2d-array, 3d textures
• test fragment stages
• test out of range values

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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.

[shrekshao]

Post some questions first

[shrekshao]

v.pos.xy is in -1, 1 range. I think you want is uint coord here (something like gl_FragCoord from 0 to tex wdith/height)?

[greggman]

v.pos.xy is in texel coordinates (it is gl_FragCoord)

   0.5, 0.5    1.5, 0.5    2.5, 0.5
   0.5, 1.5    1.5, 1.5    2.5, 1.5
   0.5, 2.5    1.5, 2.5    2.5, 2.5

[greggman]

It's confusing because

struct VOut {
  @builtin(position) pos: vec4f;
  ...
};

In the vertex shader this is normalized coord used to computing triangle vertices.
In the fragment shader it's gl_FragCoord.

[shrekshao]

Thanks for the answer. LGTM