Fix race-condition in tests against non-threadsafe scala.App

To be honest, I got the impression (from logging) that the
race-condition was occurring in the scopt parser, but I couldn't find
any evidence of thread-unsafe code in it's source.

In any case, I am sure that concurrent testing against scala.App is
definitely not safe...

https://twitter.com/rtyley/status/340376844916387840