Minor inter_stage test improvements

[kainino0x]

• Update max_shader_variable_location and max_variables_count,output in line with Alternate fix for fragment input deductions in max_variables_count,input #4555
• Add some cases of using vertex but not fragment (but not addressing maxInterStageShaderVariables test with unused variables #4564 yet)

Issue: None really, came up while looking at #4564

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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.
• Test descriptions are accurate and complete.
• Tests provide complete coverage (including validation control cases). Missing coverage MUST be covered by TODOs.
• Tests avoid over-parameterization (see case count report).

When landing this PR, be sure to make any necessary issue status updates.

[github-actions]

Results for build job (at 86c3eb0):

-webgpu:api,validation,render_pipeline,inter_stage:max_shader_variable_location:* - 6 cases, 6 subcases (~1/case)
-webgpu:api,validation,render_pipeline,inter_stage:max_variables_count,output:* - 8 cases, 8 subcases (~1/case)
+webgpu:api,validation,render_pipeline,inter_stage:max_shader_variable_location:* - 2 cases, 8 subcases (~4/case)
+webgpu:api,validation,render_pipeline,inter_stage:max_variables_count,output:* - 6 cases, 24 subcases (~4/case)
-TOTAL: 285738 cases, 2343922 subcases
+TOTAL: 285732 cases, 2343940 subcases