shader/execution: Add more constructor tests

Add abstract non-zero constructor tests. Add structure zero and non-zero constructor tests.

Issue: #1606 Issue: #1705

Requirements for PR author:

[x] All missing test coverage is tracked with "TODO" or .unimplemented().
[x] New helpers are /** documented */ and new helper files are found in helper_index.txt.
[x] Test behaves as expected in a WebGPU implementation. (If not passing, explain above.)
[x] Test have be tested with compatibility mode validation enabled and behave as expected. (If not passing, explain above.)

Requirements for reviewer sign-off:

[x] Tests are properly located in the test tree.
[x] Test descriptions allow a reader to "read only the test plans and evaluate coverage completeness", and accurately reflect the test code.
[x] 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.

gpuweb / cts