Fix deprecated override of the event loop with async-solipsism

The new version of pytest-async doesn't allow overriding the event loop
with a fixture as we used to, so we need to use the `event_loop_policy`
fixture instead.

Signed-off-by: Leandro Lucarella <[email protected]>

Author: llucax

tests/test_asyncio.py

@@ -4,7 +4,6 @@
 """Tests for the asyncio module."""
 
 import asyncio
-from collections.abc import Iterator
 from typing import Literal, assert_never
 
 import async_solipsism
@@ -13,13 +12,11 @@
 from frequenz.core.asyncio import BackgroundService
 
 
-# Setting 'autouse' has no effect as this method replaces the event loop for all tests in the file.
-@pytest.fixture()
-def event_loop() -> Iterator[async_solipsism.EventLoop]:
-    """Replace the loop with one that doesn't interact with the outside world."""
-    loop = async_solipsism.EventLoop()
-    yield loop
-    loop.close()
+# This method replaces the event loop for all tests in the file.
+@pytest.fixture
+def event_loop_policy() -> async_solipsism.EventLoopPolicy:
+    """Return an event loop policy that uses the async solipsism event loop."""
+    return async_solipsism.EventLoopPolicy()
 
 
 class FakeService(BackgroundService):