selftests/hid: tablets: move move_to function to PenDigitizer

We can easily subclass PenDigitizer for introducing firmware bugs when
subclassing Pen is harder.

Move move_to from Pen to PenDigitizer so we get that ability

Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>
Acked-by: Jiri Kosina <jkosina@suse.com>
Link: https://lore.kernel.org/r/20231206-wip-selftests-v2-6-c0350c2f5986@kernel.org


Signed-off-by: Benjamin Tissoires <bentiss@kernel.org>