If the post status is not registered, assume it's not public, but still allow access to users with edit permissions (same as for a protected post status, e.g. `draft`), so that they could recover orphaned content.
Add unit tests.
Follow-up to [47178].
Props roytanck, SergeyBiryukov.
Fixes#48653.
git-svn-id: https://develop.svn.wordpress.org/trunk@47181 602fd350-edb4-49c9-b593-d223f7449a82