test: mix tablet events into touch arbitration tests

We keep running into the proximity timeout for these tests, especially under
valgrind. To avoid this, manually intersperse the touch events with tablet
events.

Note that this manual loop would just work even without tablet events
because we no longer have a 10ms delay between touch events as enforced by
litest_touch_move_to. But let's do the right thing anyway.

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