Tests: Correct `@covers` tags for `WP_Error` tests.

Includes removing redundant tags to help clarify the purpose of each individual test.

Follow-up to [42255], [50339].

See #64225.

git-svn-id: https://develop.svn.wordpress.org/trunk@62260 602fd350-edb4-49c9-b593-d223f7449a82