Posts, Post Types: Fail gracefully when checking whether a single post with an...

Posts, Post Types: Fail gracefully when checking whether a single post with an unregistered post status should be displayed in `WP_Query::get_posts()`.

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.
Built from https://develop.svn.wordpress.org/trunk@47181


git-svn-id: https://core.svn.wordpress.org/trunk@46981 1a063a9b-81f0-0310-95a4-ce76da25c4cd