selftests/hid-tablet: add ABS_DISTANCE test for stylus/pen

selftests/hid-tablet: add ABS_DISTANCE test for stylus/pen

For pen and stylus, the ABS_Z event reports ABS_DISTANCE values
in the hid generic kernel driver. This test is to make sure that
the assignment is properly done for all pen and stylus tools.
Same as tilt, distance is an optional event.

Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Signed-off-by: Ping Cheng <ping.cheng@wacom.com>
Signed-off-by: Tatsunosuke Tobit <tatsunosuke.tobita@wacom.com>
Signed-off-by: Jiri Kosina <jkosina@suse.com>