Fix PHPUnit execution for Phing

The PHPUnitTask sometimes gives issues, there's definitely room for improvement.
I haven't pinpointed the issue you are having with it but a known "issue" is that it doesn't fully use your configuration file, therefor I recommend executing PHPUnit with the ExecTask instead.

Using the `passthru` to make the output of the command show in  your phing output and using `checkreturn` to check for the exit code and make the build fail on failed tests.

There's an upside to using the ExecTask, namely that it can switch between the *phpunit.xml.dist* and *phpunit.xml* accordingly, whereas the PHPUnitTask requires a specific XML configuration. This switching can also be done with the PHPUnitTask but it would require some extra checks.