Fixed test users explosion.

All new users created in testing are placed in "test_users" group. Additionally, added a method in the UserAdminService that first adds all users with email addresses starting with "bridge-testing" to the "test_users" group, then deletes all users in that group.
Set<String> would be better; then you can test_users without worrying if it's already there or not. And convert to an array later, wherever it was we needed it