libmm-glib,test: avoid using deprecated mm_pco_list_free()

test-pco.c: In function ‘test_pco_list_add’:
  test-pco.c:82:5: warning: ‘mm_pco_list_free’ is deprecated [-Wdeprecated-declarations]
     82 |     mm_pco_list_free (list);
        |     ^~~~~~~~~~~~~~~~
  In file included from ../../libmm-glib/libmm-glib.h:82,
                   from test-pco.c:17:
  ../../libmm-glib/mm-pco.h:67:6: note: declared here
     67 | void mm_pco_list_free (GList *pco_list);
        |      ^~~~~~~~~~~~~~~~

libmm-glib/tests/test-pco.c

@@ -79,7 +79,7 @@ test_pco_list_add (void)
         g_assert_cmpint (memcmp (pco_data, expected_pco->pco_data, pco_data_size), ==, 0);
     }
 
-    mm_pco_list_free (list);
+    g_list_free_full (list, g_object_unref);
 }
 
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