Set "use-alt-transfer-source no;" in the "mirror" system test

Set "use-alt-transfer-source no;" in the "mirror" system test

Force ns3 to use a constant source address (10.53.0.3) when sending
transfer requests for the "initially-unavailable" zone to prevent
failures of transfers not triggered by bin/tests/system/mirror/tests.sh
from causing fallback to using a source address for which transfers of
that zone are refused throughout the entire "mirror" system test since
that might yield false positives.