7.x filter-drupal.xml is writable

This is a simple addition that would be suuuuuuuper helpful for me to see at a glance if clicking the 'run tests' button is going to blow everything up.

Sometime later I'm going to make it so that you can't use IslandoraWebTestCase if you can't write to the Drupal filter, but this will suffice for now, and in the end I'd love to have both of those in.

EDIT: LATER IS NOW PEOPLE. It now refuses to run tests that rely on the Drupal filter if the Drupal filter is not writable, and disables the checkboxes of any tests inherited from IslandoraWebTestCase in this case as well.