build: Disable GLib's tests when building it as a subproject
We don't need to compile or run GLib's test suite as part of building GObject-Introspection: any failures there are not GObject-Introspection issues. This should make CI somewhat quicker.
We don't need to compile or run GLib's test suite as part of building GObject-Introspection: any failures there are not GObject-Introspection issues. This should make CI somewhat quicker.