try overriding tornasync's io_loop fixture

[minrk]

to avoid calls to deprecated make_current

closes #876

[minrk]

If there's a failure (likely a hang) it means something else is accessing the loop via IOLoop.current() or asyncio.get_event_loop() while it's not running. That ought to trigger a deprecation warning, though.

[minrk]

This is not at all pressing, since it's just avoiding a deprecated API that is still fully supported.

[blink1073]

Superseded by #1035