tests: ignore 'remove' events from udev

tests: ignore 'remove' events from udev

So far the tests have been executed on a Debian 9 machine with libudev
provided by systemd 232 - everything always worked fine. However
running them on a system with systemd 237 uncovered a bug in the
testing framework.

It turned out that on newer versions the ignored 'remove' events linger
somewhere in the pipeline and get read by subsequent test cases causing
erroneous chip naming and breaking tests.

Check the action string for udev device objects and only care about
those being added.

Signed-off-by: Bartosz Golaszewski <bartekgola@gmail.com>