Remove all test_mnist_tutorial_*.py

[carlini]

These tests are really slow to run, flakey, and should be replaced by unit tests of each individual method.

[goodfeli]

I think they should be sped up, rather than removed. They're the only thing we have resembling an integration test. Integration tests are important as well as unit tests.

[goodfeli]

Also, before we had the tests, people routinely made edits to the tutorials themselves that broke the tutorials.

[npapernot]

Given that #155 cut testing time to about 12mn, I'm going to close this for now. Once unit test have a better coverage of the code base, we can decrease the test time even more by further reducing the problem size of tutorial tests.