Update our GitHub Actions workflow with the following tests:
1. Build a .deb for Dangerzone on Debian Bookworm.
2. Install this .deb on every Debian-based platform that we support.
3. Test that the installed version runs successfully.
This way, we can be sure that .deb that we create on a single Debian
version (here we choose Debian Bookworm) works on all platforms.
Refs #358
Adds tests for macOS and Windows with the dummy converter. Tests won't
actually perform the conversion. But it should be enough for us to test
the remainder of the codebase.
Fixes#229