Automated tests for sensor views

[Flix6x]

As pointed out in PR #223, we need to add tests for our sensor views (see ui.views.sensors.SensorUI), for example, in a new test_sensor_crud.py. This may require mock API responses, as we did earlier in test_asset_crud.py.

We currently have only to read-endpoints (sensor editing still needs to be done in #433 and #434), so there would not be as much work as for assets (for now).