I have a hack in place to skip asyncio tests in Python < 3.4 (since the "yield from" syntax prevents those tests from even being parsed in older Pythons). Revisit the hack and make a reasonable effort at a more easily understood method of skipping those tests.

Also move the Tornado-specific suite into a subdirectory and skip it if Tornado is not installed.

Fail the suite if neither tornado nor asyncio is installed, to prevent false success on ImportError.