Rewrite function names from camelCase to snake_case

[The-Compiler]

When writing unittest.py tests, often the test methods are camel-cased, e.g. def testThing(self): .... With pytest, the usual convention is snake_case (as per pep8), i.e. def test_thing():.

It'd be nice if unittest2pytest could rename the functions accordingly.

[dannysepler]

borrowed this idea for pytestify!