fix race in unit test (#2018)
fix race in unit test (#2018)

fixes #1979 
klausman/scion,Oncilla/scion,Oncilla/scion,klausman/scion,Oncilla/scion,klausman/scion,netsec-ethz/scion,netsec-ethz/scion,klausman/scion,Oncilla/scion,netsec-ethz/scion,netsec-ethz/scion,klausman/scion