Mark tests that need couch

tests/couch/test_atomicity.py

tests/couch/test_backend.py

tests/couch/test_command.py

tests/couch/test_ddocs.py

tests/couch/test_state.py

tests/couch/test_sync.py

tests/couch/test_sync_target.py

tests/server/test_incoming_server.py

tests/server/test_tac.py