tests: Destroy user_user_group 'perms_group_1'

This doesn't seem to be a problem when running `nosetests`, but with `py.test` for some reason it becomes a problem:

$ py.test kallithea/tests/models/test_user_permissions_on_repo_groups.py kallithea/tests/models/test_user_group_permissions_on_repo_groups.py --maxfail=1 --tb=short
...
build/bdist.macosx-10.6-intel/egg/sqlalchemy/engine/default.py:331: in do_execute
cursor.execute(statement, parameters)
E IntegrityError: (IntegrityError) UNIQUE constraint failed: users_groups.users_group_name
u'INSERT INTO users_groups (users_group_name, user_group_description, users_group_active, users_group_inherit_default_permissions, user_id, created_on, group_data) VALUES (?, ?, ?, ?, ?, ?, ?)' ('perms_group_1', 'DESC', 1, 1, 2, '2015-04-08 17:38:03.457963', None)
...