Manage uvloop's loops & policies properly in tests

[nolar]

Otherwise, pytest-asyncio complains with deprecation warnings (only with uvloop!):

E               DeprecationWarning: pytest-asyncio detected an unclosed event loop when tearing down the event_loop
E               fixture: <uvloop.Loop running=False closed=False debug=False>
E               pytest-asyncio will close the event loop for you, but future versions of the
E               library will no longer do so. In order to ensure compatibility with future
E               versions, please make sure that:
E                   1. Any custom "event_loop" fixture properly closes the loop after yielding it
E                   2. Your code does not modify the event loop in async fixtures or tests

…/python3.11/site-packages/pytest_asyncio/plugin.py:444: DeprecationWarning

This makes no difference for the real CLI usage as it is usually used only once and then the process is closed.

Related: #998.