Automated tests for sensor views

[Flix6x]

As pointed out in PR https://github.com/SeitaBV/flexmeasures/pull/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 one sensor page which shows a chart. Editing is possible in the API, but not supported in the UI (see #434), so there would not be as much work as for assets (for now).