Modernize event loop handling in aiohttp tests

[musicinmybrain]

Issue #, if available:

N/A

Description of changes:

This fixes compatibility with pytest-aiohttp 1.1.0.

First, migrate aiohttp tests from loop fixture to event_loop. This follows advice from an upstream deprecation warning, and furthermore, as of pytest-aiohttp 1.1.0 these tests break since the loop fixture is no longer available at all.

Then, in async tests, replace event_loop fixtures with calls to asyncio.get_running_loop(), as advised by, e.g.:

tests/ext/aiohttp/test_middleware.py:123
  tests/ext/aiohttp/test_middleware.py:123: PytestDeprecationWarning: test_ok is asynchronous and expl
icitly requests the "event_loop" fixture. Asynchronous fixtures and test functions should use "asyncio
.get_running_loop()" instead.
    async def test_ok(aiohttp_client, event_loop, recorder):

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