Since GLib 2.73.0 it can only be called once per process.
Based on commit 78c153ae
in GTK 4, but combining the icontheme test's
two custom log writers into one log writer that serves both purposes,
to avoid the need for a subprocess.
Co-authored-by: Matthias Clasen <mclasen@redhat.com>
Resolves: https://gitlab.gnome.org/GNOME/gtk/-/issues/5119