Rename PHPUnit config file from phpunit.xml to phpunit.xml.dist
* PHPUnit supports config files with both these names, but will use phpunit.xml over the .dist version.
* Renaming our file lets users add/edit their own phpunit.xml without editing a bbPress core file.
* Hattip djpaul via [BuddyPress:r8098]
* Props netweb. See #2542

git-svn-id: https://bbpress.svn.wordpress.org/trunk@5323 9866e705-20ec-0310-96e7-cbb4277adcfb