Fix test_run_task

Using `with trio_asyncio.open_loop` (instead of async with) fails with
`AttributeError: __enter__` on Python 3.7+.

Author: pquentin

tests/test_misc.py

@@ -167,7 +167,7 @@ async def nest(x):
         with pytest.raises(RuntimeError):
             trio_asyncio.run_trio_task(nest, 100)
 
-        with pytest.raises(RuntimeError):
+        with pytest.raises((AttributeError, RuntimeError)):
             with trio_asyncio.open_loop():
                 nest(1000)