Make test helpers reusable by other projects

Puppet Server has a number of test helper functions which would be useful in other projects - namely, pe-puppet-server-extensions. These should be exposed in way that they can be consumed by other projects.

The preferred solution here is to simply build a test .jar, the way our other Clojure projects do (e.g. Trapperkeeper). This is slightly more complicated in this case, since Puppet Server uses voom to do intermediate/internal releases - but chris says that it should work just fine.