The tests `core.sigterm-regular` and `core.sigterm-stdin` rely on `ps`
to be present which is not the case anymore on OpenSuse Leap; install it
explicitly there.
This also adds 15.4 to the list of distributions that are skipped by
default - let me know if this is not a good idea. I assume that by now
most people will be running 15.5 (15.4 will be EOL in ~3 months).