Updates `wp_get_active_and_valid_themes()` to return early when `wp_using_themes()` returns `false`. This prevents a theme's `functions.php` from being loaded erroneously when the site isn't using themes.
Also adds `define( 'WP_USE_THEMES', true );` to the test suite bootstrap. Some tests randomly break without it because they were dependent on the previous buggy behavior.
Props bpayton, costdev, danielbachhuber, hellofromtonya, sergeybiryukov, spacedmonkey.
Fixes#57928.
git-svn-id: https://develop.svn.wordpress.org/trunk@55890 602fd350-edb4-49c9-b593-d223f7449a82