Make dependency for phpunit config configurable

[e42sh]

When using ant generate-reports there is a hard dependency for app/phpunit-report.xml. We should make it configurable, since it differs from symfony standard edition and any other project that will use the ant tools.

OK I see that there is another issue. ant build uses the default dist file php-unit.xml.dist that is offered in symfony standard edition, but ant generate-reports expects app/phpunit-report.xml.