test: switch event conversion tests to use litest devices

test: switch event conversion tests to use litest devices

Makes the code use more commonly used paths, no real functional changes at
this point. This was using hand-crafted devices as it predates the
litest_add_for_device() helper.

For an upcoming patch to use the udev ID_INPUT_. tags the
event_conversion_key test requires this change: without it the device will be
tagged with ID_INPUT_KEY but not ID_INPUT_KEYBOARD. This could be fixed by
adding all normal keyboard keys to the uinput device but it's easier to just
re-use litest.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>