There were not checks in test.libregrtest.save_env to make sure tests were not mucking with the default event loop. The settings to check seem to be:

'asyncio.get_event_loop_policy', 'asyncio.get_event_loop',
'asyncio_get_exception_handler', 'asyncio_get_debug',
'asyncio_get_child_watcher',

I have a PR that I'm working on that checks these things and it looks like the following tests are being a little sloppy:

test_asyncgen
test_contextlib_async
test_coroutines
test_socket
test_sys_settrace

And test_asyncio never seems to complete.

New changeset 8425de4147eb8d83befbb8ea77516fc764bb4309 by Brett Cannon in branch 'master':
bpo-33562: Check the global asyncio event loop policy isn't set after any tests (GH-7328)
https://github.com/python/cpython/commit/8425de4147eb8d83befbb8ea77516fc764bb4309