Tests: Add coverage for paused-theme recovery helpers.

Cover paused-theme detection, stored error retrieval, and resuming themes. Verify error removal preserves other paused themes, inactive themes do not redirect, and active themes load their `functions.php` file while discarding its output and generating a valid error nonce.

Developed in: #14162

Props huzaifaalmesbah.
See #65819.


git-svn-id: https://develop.svn.wordpress.org/trunk@64414 602fd350-edb4-49c9-b593-d223f7449a82

Author: lancewillett