Tests

While there technically are some cabal `test-suite` stanzas ([1](https://github.com/tomsmeding/play-haskell/blob/master/play-haskell-server/play-haskell-server.cabal), [2](https://github.com/tomsmeding/play-haskell/blob/master/play-haskell-worker/play-haskell-worker.cabal)), they require quite a bit of manual involvement to use properly, and don't really test a lot.

Easy-to-run integration tests, as well as property tests for the internals when applicable, would be quite welcome!