Tests: Cached data in `WP_Theme_JSON_Resolver` causes cross pollution in unit tests

While working on #59595, @thekt12 and I discovered that while all of the tests in the Tests_Theme_WpAddGlobalStylesForBlocks class were passing when run together, there were a few that failed when run in isolation, specifically:

• test_blocks_inline_styles_get_rendered()
• test_third_party_blocks_inline_styles_for_elements_get_rendered()

In both cases, the tests were relying on cached data that had been set up in previous tests by calling $this->set_up_third_party_block(); to still be returned by the WP_Theme_JSON_Resolver class even though these tests don't call $this->set_up_third_party_block() themselves.

To confirm, run the following to see all tests in the class pass:

npm run test:php -- --filter Tests_Theme_WpAddGlobalStylesForBlocks

And the following to see an example of a test that fails in isolation

npm run test:php -- --filter test_blocks_inline_styles_get_rendered

The reason this happens is that the theme gets reset between each test that extends the WP_Theme_UnitTestCase but the wp_clean_theme_json_cache() function was not being called to clean Theme JSON related caches (including the WP_Theme_JSON_Resolver::clean_cached_data() method).