Add E2E tests for the nav menu toggle

[westonruter]

Feature description

The 2.0.2 update broke the ability to toggle the mobile nav menu. See #5401 fixed by #5403. Given that the nav menu toggle is the most important interactive feature that almost all themes have, and given that it is the feature that almost always is broken when a theme is not AMP-compatible, we need to add E2E tests to ensure that this toggle works. This should be done for a theme that uses the AMP_Nav_Menu_Toggle_Sanitizer like Twenty Seventeen but also for themes that have their own implementation, like Twenty Twenty. While the regression was actually a fault with the CSS sanitizer and would have been caught by more extensive tests, the issue would have also been caught by an E2E test which would also ensure all the classes in between are also working properly, including the AMP_Core_Theme_Sanitizer (see #3729).

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Acceptance criteria

Implementation brief

QA testing instructions

Demo

Changelog entry

[westonruter]

Re-opening because there's a failure for the Twenty Twenty test:

[westonruter]

Aside: if it's feasible to add a test for Twenty Twenty-Two as well, that would be great.