Add a test to see whether the produced runtime works

[probonopd]

We need to add a test to see whether the produced runtime works. We had this in AppImageKit.

Currently the build can be green even though the runtime segfaults immediately.

This might have prevented

• Runtime segfaults #51

This might partially be accomplished by

• Trigger appimagetool build #50

and then testing the produced appimagetool AppImage briefly there.