Fix test for new seqno when reacting to a route retraction.
Fix test for new seqno when reacting to a route retraction.

wlanslovenija/babeld,Gwendocg/babeldToS,dtaht/babeld-shortrtt-metrics,tcatm/babeld,jech/babeld,Drooids/babeld,wlanslovenija/babeld,Gwendocg/babeldToS,sudomesh/babeld,boutier/babeld,jech/babeld,woniullb/babeld