Test Twenty Twenty search modal on mobile breakpoint

[delawski]

Summary

Fixes #5404

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

As per #5404 (comment), the E2E tests that were recently added in #6933 are failing for Twenty Twenty theme.

This PR fixes the failing test. Instead of examining a desktop search modal, we're now testing a mobile search modal.

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

Also, the request for adding E2E tests for Twenty Twenty-Two theme navigation block in the header has been addressed here (0d59631).

Checklist

• My code is tested and passes existing tests.
• My code follows the Engineering Guidelines (updates are often made to the guidelines, check it out periodically).

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[westonruter]

Thank you!