use the qapp fixture and `qapp.processEvents()`

[joshmoore]

          Ah, I think but don't know that you can instead use the qapp fixture and do `qapp.processEvents()`, see e.g. https://github.com/napari/napari/blob/157b144d90e0a605a5dac68321dcd34250cd6590/napari/_vispy/_tests/test_image_rendering.py#L58-L74

Originally posted by @jni in #112 (comment)