flatpak: Fix self tests RuntimeRepo warning
Flatpak now emits a warning when RuntimeRepo isn't present in a flatpakref, so add it to fix the unit tests. See https://github.com/flatpak/flatpak/pull/2151
Flatpak now emits a warning when RuntimeRepo isn't present in a flatpakref, so add it to fix the unit tests. See https://github.com/flatpak/flatpak/pull/2151