tests: Fix a double-unref in params tests

This isn’t normally hit because it’s in a test which is disabled unless
run with `-m thorough`.

The `GParamSpec` is initially floating, but its floating ref is sunk by
`g_object_interface_install_property()` (regardless of whether that call
succeeds or aborts). The behaviour of
`g_object_interface_install_property()` in this respect may have changed
more recently than the test was written.

Signed-off-by: Philip Withnall <pwithnall@gnome.org>

gobject/tests/param.c

@@ -1157,7 +1157,6 @@ test_interface_default_init (TestInterfaceInterface *iface)
             g_object_interface_install_property (iface, pspec);
             g_test_assert_expected_messages ();
 
-            g_param_spec_unref (pspec);
             continue;
           }