Tests are failing with napari 0.5.0

[adamltyson]

@alessandrofelder's hypothesis - "napari fixtures are now cleverer and fail your tests if you don't clean up your widgets properly"

We should look into this and get the tests passing again (e.g. by making sure all qt widgets have parents)

[alessandrofelder]

Tests marked with xfail and the appropriate error in #213 as a workaround until this is figured out on the napari side.

[alessandrofelder]

One xfail here has now been successfully removed by #260.
The other needs more investigation, so keeping this open for now.