Fix database schema update introduced in [30121].

The index must be manually dropped before `dbDelta()` can add the new index
without throwing a notice.

Fixes #22023.

git-svn-id: https://develop.svn.wordpress.org/trunk@30134 602fd350-edb4-49c9-b593-d223f7449a82
Fix failing unit tests after [30096]: use `assertEqualSets()`.



git-svn-id: https://develop.svn.wordpress.org/trunk@30135 602fd350-edb4-49c9-b593-d223f7449a82