(#143) - Add express-pouchdb specific test suite

A test suite setup for express-pouchdb specific. Why?

- The first test is actually a test that would have found out #139 sooner. test-pouchdb & test-couchdb both would never have found it, because it's express-pouchdb specific functionality.
-  @yaronyg wants to add tests for  #125. Until this PR, there would've been only two options: maintaining tests out-of-tree or unblocking enough of couchdb-harness (with the last being impractical, and the first what I actually do for a lot of my plug-ins). Now there's an alternative, adding them here.

Oh, I also hope to add a few express-pouchdb specific options in the future. Like the location of the config file (currently hardcoded to ./config). No other place for tests for that to go either.