The format of the filename in junit xml file can be specified. This fixes drush-ops/drush#50.

There is an option 'test-results-filename-format', which can have
the values 'absolute', 'relative' and 'basename' and defaults to
'basename'. Thus the current behavior is conserved, while relative
or absoltue filenames can be inserted via drushrc.php

Todo: Where does the documentation of this option go?