tests: Fix a double-unref in the GSettings unit tests
tests: Fix a double-unref in the GSettings unit tests

g_settings_schema_source_get_default() is (transfer none), not (transfer
full).

Spotted by Marvin Schmidt.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

https://bugzilla.gnome.org/show_bug.cgi?id=779265

endlessm/glib,johne53/MB3Glib,endlessm/glib,endlessm/glib,johne53/MB3Glib,endlessm/glib,johne53/MB3Glib,johne53/MB3Glib,johne53/MB3Glib,johne53/MB3Glib,endlessm/glib