Testmode bug fix

Fixes https://trello.com/c/EgW3usAa/191-bug-testmode-doi-generation

Testmode DOIs don't need to be that complicated: according to http://ezid.cdlib.org/demo/simple, 10.5072/FK2. will suffice for making a demo DOI.
This change would remove any dryad-specific naming convention from the test DOIs. 

the doi.localpart.testsuffix would typically be `FK2.5061.dryad.`, so if we change that to just `FK2`, we could have collisions on test mode, right? I chose to keep 5061 since that's our official prefix, but I think we should at least include dryad, since FK2 would be used by everyone that uses EZID.