tests: Fix a minor leak in the cxx string append tests
See https://gitlab.gnome.org/GNOME/glib/-/jobs/2547640
Signed-off-by: Philip Withnall pwithnall@endlessos.org
Helps: #2904 (closed)
See https://gitlab.gnome.org/GNOME/glib/-/jobs/2547640
Signed-off-by: Philip Withnall pwithnall@endlessos.org
Helps: #2904 (closed)