`test_app_dragged_around` not resilient to different resolutions

[Saviq]

Running under a different resolution may prevent the test from working:

$ MIR_SERVER_X11_OUTPUT=1920x1080 pytest 'test_screencopy_bandwidth.py::TestScreencopyBandwidth::test_app_dragged_around[mir_test_tools]'
# ...
E       AssertionError: expected to capture at least 16 frames, but only got 2
E       assert 2 >= 16