Do away with `combo_check` and switch to test parametrization

[agriyakhetarpal]

the combo_check function predates many improvements made in testing infrastructure over recent years, and is quite dated now. I propose that we refactor the test suite to use pytest better and pave the way for it to be nicer for maintainers and new contributors.

If we were to deprecate the combo_check function and change to a few simple pytest parametrization decorators, we would know exactly what sub-test fails in a test, and where – i.e., which would better isolate the failure instead of us having to do it ourselves.

from @agriyakhetarpal in #643 (comment)

I agree with this idea. Presumably we should be able to get roughly equivalent tests using pytest parameterized tests. If you do implement this it might be worth checking that it doesn't slow the tests down (I like the fact that our tests are really fast).

from @j-towns in #643 (comment)

A task list

• reimplement combo_check in terms of pytest fixtures and parametrized test methods
• parametrize tests where possible and where combo_check is used
• make autograd.test_util a private API where possible