fullyExpandSubtrees & ability to disable no long path assert

[sagudev]

Assert is not valid (at least in servo's case) as only one expectation file is used for all variants. Another option would be to reduce assert to warning (depending on how much it is needed elsewhere).

fullyExpandSubtrees expends all subs recursively that are part of provided query.

Issue: #3233

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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.

[kainino0x]

LGTM, thank you for the revision!