Prevent an uncaught exception warning in a test suite thread

[kurtmckee]

Spotted this while attempting to run the test suite locally with warnings enabled:

tests/middleware/test_asyncio.py::test_event_loop_thread_start_timeout
  lib/python3.12/site-packages/_pytest/threadexception.py:77:
    PytestUnhandledThreadExceptionWarning: Exception in thread Thread-13

The easiest fix is to stop raising an exception in the mock. In addition, since RuntimeError may be raised in several ways, I added a match argument to the pytest.raises() context manager to identify which RuntimeError text is expected.

After avoiding that warning, a ResourceWarning popped up:

tests/middleware/test_asyncio.py::test_event_loop_thread_start_timeout
  lib/python3.12/asyncio/base_events.py:726:
    ResourceWarning: unclosed event loop <_UnixSelectorEventLoop running=False closed=False debug=False>

This happens because the underlying event loop (the one that is getting swapped out with a mock) is not getting closed.