Promote inference tests into the common test suite

[joshsh]

Currently we have inference tests only in Haskell, because we have inference only in Haskell. This will change, and we will need the tests in Java and Python as well, to ensure parity. Extend hydra/tests to include inference tests with flags / tags (so we can selectively disable certain tests in certain contexts), and move the test cases from the *Spec.hs files into the DSL.

[joshsh]

Note: generative tests will have to remain in Haskell for now. However, most of the individual test cases can be promoted.