flatpak: Do not remove flatpak/tests/app-update/repo content
The "/gnome-software/plugins/flatpak/app-update-runtime" test removes "_build/plugins/flatpak/tests/app-update/repo" directory content, through (glib's testing facility), thus when trying to run the test again, without calling "make", causes failure of this test due to the repository not having any content (and index, as is printed in the test log).