bpo-23819: Get rid of assert statements in test_asyncio (GH-30212) (GH-30213)

To keep checks even if run tests with optimized Python.

Either use special assertion methods like assertEqual() or
raise an AssertionError explicitly.
(cherry picked from commit 6ca78affc8023bc5023189d64d8050857662042a)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>