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

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>