tests: fix incorrect free of GVariant in our GLib mock functions

GLib implementation of g_dbus_connection_call_sync() calls
g_variant_ref_sink() on the passed @parameters to make sure they have
proper reference. If the original reference is floating the
g_dbus_connection_call_sync() consumes it, but if it's normal reference
it will just add another one.

Our mock functions were only freeing the @parameters which is incorrect
and doesn't reflect how the real implementation works.

Reported-by: Cole Robinson <crobinso@redhat.com>
Signed-off-by: Pavel Hrdina <phrdina@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>

tests/virsystemdtest.c

@@ -54,8 +54,10 @@ VIR_MOCK_WRAP_RET_ARGS(g_dbus_connection_call_sync,
 {
     GVariant *reply = NULL;
 
-    if (parameters)
+    if (parameters) {
+        g_variant_ref_sink(parameters);
         g_variant_unref(parameters);
+    }
 
     VIR_MOCK_REAL_INIT(g_dbus_connection_call_sync);