test: fix the slot swap test again

The previous movement was one finger still, the second finger moving. This may
cause axis events to trigger when a 2fg scroll gesture was detected. Those
axis events will stop after the gesture timeout but generate one more axis
stop event.

Make two changes here: first, move the fingers like a proper 2fg scroll
motion. And shuffle around the litest_drain_events() calls to ignore any axis
event immediately after the timeout.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>