In test/fixtures.py there are several fixtures that create a bunch of Ursulas: fleet_of_highperf_mocked_ursulas, blockchain_ursulas. After creation they are given to each other to learn about circumventing the standard channel (REST API). As a result:

• FleetSensors own fully-functional Ursulas, and not is_me=False remote images. This can result in different test and production behavior, and possible side effects (since is_me=True Ursulas can mutate their internal state, which FleetSensor does not expect to happen).
• Ursulas are shared between FleetSensors, which can also interfere with test behavior (since, again, these Ursulas are mutable).