[r=axwalk],[bug=1282692] provider/common: fix time-sensitive test
[r=axwalk],[bug=1282692] provider/common: fix time-sensitive test

Test unnecessarily uses a goroutine; we
can just prime the "interrupted" channel
before calling into WaitSSH.

Fixes lp:1282692

https://codereview.appspot.com/97730043/
reedobrien/store