This functionality was originally added in [38858], using `symlink()` to put a link to the theme in WordPress' `themes` directory. Unfortunately, not all installs have write access to the `themes` directory, causing unit tests to fail.
The new method is to add the test theme directory to `$wp_theme_directories`, and fix the handful of tests that don't expect `$wp_theme_directories` to have multiple entries.
The test install/bootstrap routines now also check that `WP_DEFAULT_THEME` is defined, in case the tests are being run on a system that hasn't upgraded its' `wp-tests-config.php`.
See #31550.
Fixes#38457.
git-svn-id: https://develop.svn.wordpress.org/trunk@38907 602fd350-edb4-49c9-b593-d223f7449a82