Clear theme json data between theme unit tests

[joemcgill]

This clears the theme json related caches in the WP_Theme_UnitTestCase:::tear_down() method to avoid cross pollution between tests when a theme is changed during a test run and fixes a few tests that were only passing by relying on this cross pollution.

Trac ticket: https://core.trac.wordpress.org/ticket/61337

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Props joemcgill.

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[tellthemachines]

Thanks for spotting this! The change makes sense and the tests are now passing when run individually. Fwiw I couldn't reproduce the failure when running npm run test:php -- --filter test_third_party_blocks_inline_styles_for_elements_get_rendered on trunk, but it's reproducible on this branch if I remove $this->set_up_third_party_block(); from the test. LGTM!

[joemcgill]

Merged in https://core.trac.wordpress.org/changeset/58295