Simplify PHPUnit setup.

 * Use the much more succinct style of composing suites with XML configurations.
 * Remove all unnecessary includes/requires. All test runs should load the bootstrap which includes the appropriate autoloader.
 * Move the XML configuration file to a more standard location so that PHPUnit can run from the root directory with no arguments.
 * Allow for local phpunit.xml files.
 * Use the version of PHPUnit that's installed by composer.