build: Re-add libglib-testing subproject
This time, add it as a wrap module rather than a git submodule. They’re easier to manage, and integrate better with Meson.
The subproject has to be re-added so that malcontent can be built on Debian Stable and Fedora 31 for the gnome-software CI. See https://gitlab.gnome.org/GNOME/gnome-software/-/merge_requests/487.
Signed-off-by: Philip Withnall withnall@endlessm.com