Add basic tests in packit

[ekohl]

This uses tmt to run basic tests on packit builds.

Inspired by #918 without duplicating the whole test setup. I've chosen tmt for its integration in packit. It would be interesting to see if this can also be used standalone with a development install.

[ehelms]

Do you have any docs on what tmt is, why we want to use it and where we draw the line between GHA and whatever tmt is?

[ekohl]

It utilizes tmt and Packit in turn uses Testing Farm to run those. The idea is that you run some tests after you've built your package on a real system. If this works, we can give a much better guarantee our packages actually work. Unlike the %check phase in RPM, this runs on actually machines (containers or VMs).

[nforro]

@ekohl You need to set use_internal_tf: true on the tests job if you want to use RHEL composes.