This takes into account plugin or theme test suites that rely on `WP_PLUGIN_DIR` being set to a custom path.
Follow-up to [49236], [49269], [49566], [53874].
Props aaemnnosttv, flixos90.
Fixes#39210.
git-svn-id: https://develop.svn.wordpress.org/trunk@53902 602fd350-edb4-49c9-b593-d223f7449a82