Generate JUnit xml for failed dependant tests

[OndraM]

If test fails because its prerequisite test failed before, it is marked as failed in results.xml, however, its junit xml is not generated at all (obviously because the phpunit process is not even started).

This confuses junit plugins like the one in jenkins, because it has no way how to know about this failed tests.

One possible solution is to generate some "dummy" xml in junit format (and with proper name) for this failed tests, with error message like Testcase "Foo\DependantTest" marked as failed because it was depending on failed "Foo\FailedTest".

The previous xml files should be cleared before Steward starts, so if the file already exists, it probably should not be overriden (or should with warning?).