[tests] Fix: always run test cases
[tests] Fix: always run test cases

Previously, python2 builds were always enabled and the general test cases
were run as part of (also) building the python2 buildings.

Recently, though, python2 builds have been disabled by default on newer
distros (due to deprecation), but that also means that the general test
cases are not run any longer.

Fix that by running the general test cases in python3 builds if python2
builds are disabled.

Closes: #782
Approved by: jrohel

rpm-software-management/libdnf,hughsie/libhif,rpm-software-management/libdnf,rpm-software-management/libhif,rpm-software-management/libdnf,rpm-software-management/libhif,hughsie/libhif,rpm-software-management/libhif,hughsie/libhif,hughsie/libhif,rpm-software-management/libhif,rpm-software-management/libdnf