Discard shader execution tests

[alan-baker]

• Tests:
  • discard all
  • discard 3 quadrants
  • discard in function call
  • discard in various loops
  • derivatives and discard

Issue: #1683

---

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.

[jrprice]

Perhaps I'm missing something, but the shader logic and/or predicates seem off for a couple of tests.

[jrprice]

The fragment positions LGTM now, but I'm still a little concerned about the fact that a couple of the tests don't fail if you make discards unconditional.

[jrprice]

LGTM, thanks!