Fix tests for Python 3.14

[musicinmybrain]

Use the event_loop fixture for tests that need one.

Without this, many tests fail in Python 3.14.0a2 because asyncio.get_event_loop() no longer automatically starts an event loop, instead raising a RuntimeError if there is no current event loop. See https://docs.python.org/dev/library/asyncio-eventloop.html#asyncio.get_event_loop, https://docs.python.org/dev/whatsnew/3.14.html#id3, and python/cpython#126353.

For documentation on the fixture, see https://pytest-asyncio.readthedocs.io/en/latest/reference/fixtures/#event-loop.

Additionally, configure asyncio_default_fixture_loop_scope to function, which will be the default in a future release of pytest-asyncio. This isn’t specifically required for Python 3.14, but it fixes warnings like this:

PytestDeprecationWarning: The configuration option
"asyncio_default_fixture_loop_scope" is unset.  The event loop scope for
asynchronous fixtures will default to the fixture caching scope. Future
versions of pytest-asyncio will default the loop scope for asynchronous
fixtures to function scope. Set the default fixture loop scope
explicitly in order to avoid unexpected behavior in the future. Valid
fixture loop scopes are: "function", "class", "module", "package",
"session"