Use C.UTF-8 instead of en_US.UTF-8 when running the test cases
Because of
https://fedoraproject.org/wiki/Changes/Remove_glibc-langpacks-all_from_buildroot
Add desktop-file-utils, python3-gobject, python3-gobject-base, gtk3,
dconf, dbus-x11, ibus to BuildRequires, this seems to be not in
the chroot by default anymore in f29 and rawhide.