Add frag_depth qualifier execution test

[dj2]

Adds an execution test that the greater and less qualifiers are accepted on the frag_depth builtin.

Issue: gpuweb/gpuweb#5342

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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 7b2e841):

+webgpu:api,operation,rendering,depth:fragment_depth_qualifiers:* - 2 cases, 2 subcases (~1/case)
-TOTAL: 285337 cases, 2346437 subcases
+TOTAL: 285339 cases, 2346439 subcases