Improve the sensitivity of the array_zero_value tests

[andyleiserson]

In the cases where valid = true, we want to test with a const declaration, to ensure that the constructor is supported in const exprs. But in cases where valid = false, some of the subject types are not constructible, and may be rejected in a const declaration for that reason, even if the implementation is not properly restricting the use of zero-value constructors.

This changes the test to use const when valid = true and let when valid = false, which make the the tests more sensitive to the functionality they are intending to cover.

Tests still pass in Chrome. The valid_array case is failing in Safari either with or without this change. With this change, the invalid_rta, invalid_override_array, and invalid_atomic cases fail in Firefox, which I believe accurately reflects defects in the implementation.

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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]

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