build: make tests optional
Add a meson option -Dtests and --without-tests automake option to disable the compilation of all available testcases. This is useful for compiling projects with Flatpak such as GNOME Control Center which disables all possible integrations since they only need the DBus part of ModemManager.
Contributes to https://gitlab.gnome.org/GNOME/gnome-control-center/-/merge_requests/1392