Enablement for Darwin tests

[Gregory-Pereira]

I would like a set of tests that work for Darwin, currently none of the model_server or recipe tests work on Darwin. The testing infrastructure related pieces of this are discussed in #264. This issue is an outcome of closing #213, we will just have to test by hand until we can pull it into a sprint and prioritize it.

/cc @lmilbaum, this touches the initial pytest_container implementation you setup.