Unit Tests: Fix skipped Sync test

To be able to unblock Jetpack's PHPUnit tests on master, `WP_Test_Jetpack_Sync_Post::test_embed_shortcode_is_disabled_on_the_content_filter_during_sync` was skipped in https://github.com/Automattic/jetpack/pull/21030. We should fix the test.

I'd like to note, that the test is passing when running as part of the sync suite (i.e. `jetpack docker phpunit -- --filter=WP_Test_Jetpack_Sync_Post`), but failing when the whole set of tests are running.