Use faster direct call of train/predict methods in test scripts.

[jstjohn]

          TODO Dorota - this test would execute much faster is we import `train` method and just tests it. Get this unit test from gitlab

Originally posted by @dorotat-nv in #694 (comment)

[jstjohn]

There are two functions in Evo2 to apply this to. test_train.py and test_predict.py. We should make @pytest.mark.slow variants of these that test the full e2e command line call (how it goes now) but make regular versions of this test that cover the case of directly calling the function called by the CLI script.