Use the Composer autoloader when running tests

When running the tests, Agavi does not use composers autoloading. On a machine that doesn't have PHPUnit installed globally, this leads do an immediate fatal error when the first PHPUnit class is needed.

A check for composers autoload.php and a ``require`` on that file needs to be added to the test scripts (e.g. in ``test/run-tests.php``) and also in ``src/testing/templates/TestCaseMethod.tpl`` since the code generated from that template gets executed in a new process and also needs autoloading for the vendor libs.

**On TravisCI this issue does not appear because PHPUnit is installed globally on their VMs.**