Assert that testcases are not empty

[A5rocks]

I've seen new contributors make this mistake, though I can't find a PR as an example.

[bzoracler]

Would it be helpful to also display the test case name in the AssertionError?

[A5rocks]

This makes the test case fail, so pytest already does. I can do that anyways?

[bzoracler]

Oh okay, that makes sense. I would find it helpful to display it in AssertionError if pytest's output of the test case failure is far away from the assertion error; if it's not far away, then you're right, it's unnecessary.

I don't remember what test case failures look like in various pytest verbosity modes and in various IDE test runners, though.