gistfile1.txt

Good news, I've found the problem, a simple race condition.  

The issue was in Promise.scala, the test was finishing before the thread 
that had initialised the promise subsystem had had a chance to set the 
underlyingSystem Option, so when the test 
shutdown code went to shut down the promise system, it found no system to shutdown.  

Synchronizing all access to that global var solved the problem.