wgsl: compound statement tests

[dneto0]

wgsl: Add compound statement tests

Validation:
- Check parsing of compound statements
- Update break-if statement to add a case for a plain compound statement
  surrounding a break-if.
- For a declaration in a compound statement, check potential uses
  and potentially conflicting declarations.

Execution:
  Add execution tests for compound statement.

  Observe a value at various points relative to its
  declaration inside a compound statement.

Add timings for new tests

Fixed: #1634

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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.)

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.

[dneto0]

Works on NV and Apple