Tests: rename PHPUnit config file 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 BuddyPress core file.


git-svn-id: https://buddypress.svn.wordpress.org/trunk@8098 cdf35c40-ae34-48e0-9cc9-0c9da1808c22