The opposite of calling assertRaises is simply calling the code. A comment as to why you're calling the code is sufficient documentation as to the intent of the test.

In general the difference be a test failure and error is not useful - you *still* have to look at the traceback (failure details) to see *why* the test failed / errored. I'm sure the distinction comes from JUnit that unittest was originally based on (and yes - overengineered for Python).