Simplify test_cases structure

[sravanareddy]

Instead of the nested directories and CONFIG files to be generated by testCaseReader.py, allow us to specify test cases as a single JSON (?) file that will be read and interpreted by autograder.py. I think it's mainly going to be a change to evaluate and getTestSubdirs.

@dkreimer postpone the randomness testing for now -- can you work on this in the next week?

[sravanareddy]

Hey @dkreimer , will you be able to work on this? Even tackling some part of it will be helpful.

Everything is going to be handled by otterInspect.py moving forward, so the changes will made to evaluate and getTestSubdirs in that file.

(I've just created a development branch so ongoing development doesn't conflict with what we put out to students. Please commit all major changes there.)