Add unit tests to check the new feature to support multiple installations
Add unit tests to check the new feature to support multiple installations

Test all the new public APIs, making sure that the list of system-wide
installations is returned in the right order according to their priorities.

flatpak/flatpak,matthiasclasen/flatpak,flatpak/flatpak,matthiasclasen/flatpak,handsome-feng/flatpak,handsome-feng/flatpak,handsome-feng/flatpak,matthiasclasen/flatpak,GeorgesStavracas/flatpak,GeorgesStavracas/flatpak,GeorgesStavracas/flatpak,flatpak/flatpak,matthiasclasen/flatpak,GeorgesStavracas/flatpak,GeorgesStavracas/flatpak,flatpak/flatpak,handsome-feng/flatpak,matthiasclasen/flatpak,handsome-feng/flatpak,flatpak/flatpak