Add a LIBINPUT_TEST_DEVICE udev parameter to test devices created by
the test suite. When an application tries to add such a device to the
path backend or when the udev backend discovers such a device, it will
be ignored. Only the context when run via the test suite will actually
handle these devices.

Doing this will enable a user to run the libinput test suite on a system
running libinput without having the test suite devices interfering with
the actual system.

Note that X.org users running an input device driver that is not the
libinput X input driver will still need to manually configure the X
server to ignore such devices (see test/50-litest.conf).

Signed-off-by: Jonas Ådahl <jadahl@gmail.com>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>