use asyncio.Runner for tests

use asyncio.Runner for tests

Backported to SQLAlchemy 2.0 an improvement to the test suite with regards
to how asyncio related tests are run, now using the newer Python 3.11
``asyncio.Runner`` or a backported equivalent, rather than relying on the
previous implementation based on ``asyncio.get_running_loop()``.  This
should hopefully prevent issues with large suite runs on CPU loaded
hardware where the event loop seems to become corrupted, leading to
cascading failures.

Fixes: #11187
Change-Id: I867b2478b9ba3a152fbfef380650eb987527ba46