wacom: Add scenario tester

Running the helper will launch gnome-control-center with a mocked Wacom tablet through umockdev when one isn't present or easily available.

@feaneron this should help with creating a mock tablet, but we'll need to tweak one of the calls in libwacom, see: https://github.com/linuxwacom/libwacom/issues/438

@whot @carlosg you wouldn't happen to have ready-made umockdev devices I could use for this that has button mapping support? Otherwise I'll redo this from a udev dump I have. Any ideas on how to mock the stylus too?