cs: fix testing race (#3920)
cs: fix testing race (#3920)

Fix racy access to MAC object in the originator test.
This is hitting us only now, because the originator now does parallel
beacon origination to avoid HOL blocking.
netsec-ethz/scion,Oncilla/scion,netsec-ethz/scion,Oncilla/scion,netsec-ethz/scion,netsec-ethz/scion,Oncilla/scion,Oncilla/scion