meson: force litest to use the right udev rules

We generate litest-specific udev rules that contain the path to the binaries
in the builddir. But litest wasn't using those, so IMPORT would run things in
/usr/lib/udev instead. Thus any changes to those binaries generated false test
results depending on how compatible the system-installed libinput was.

This is why 410b157e passed the test suite for example.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>