Port test to Qt5/KF5 and make it use Qt Test

It was disabled in 2011 when doing some dbus related changes.
Fortunately, kmix is organized in libraries nowadays and it's much
easier and quicker to build the test.

To be honest I'm not entirely sure it's a meaningful test, though,
but I guess it's better than nothing.
2 jobs for !15 with work/port-test-to-qt5 in 1 minute and 37 seconds (queued for 50 minutes and 4 seconds)
latest merge request