Renames PHPUnit config file to default expectation

:information_desk_person: As per [the documentation on PHPUnit's `---configuration` command line switch](http://phpunit.de/manual/current/en/textui.html):

> If `phpunit.xml` or `phpunit.xml.dist` (in that order) exist in the current working directory and `--configuration` is not used, the configuration will be automatically read from that file.

This change renames the current configuration file to meet this expectation.