fix tests

[williamFalcon]

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Fixes # (issue).

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[MattPainter01]

The test I put also tries to run a test when only test_end is implemented. Should just be able to move that test inside a raises MisConfigException to fix these errors, if we require test_step.

Specifically, this line:
https://github.com/PyTorchLightning/pytorch-lightning/blob/6b667b1237f8ed74db920a2c1c49d55ca1d2b7e5/tests/test_trainer.py#L809