create a new event_loop fixture

pytest-dev/pytest-asyncio#278 allows for a parametrized event_loop fixture; let's try using this to see if we can avoid a closed event loop error.

tests/conftest.py

@@ -59,6 +59,14 @@ async def reportFailed(self, *args, **kwargs):
         raise taskcluster.exceptions.TaskclusterRestFailure("foo", None, status_code=self.status)
 
 
+@pytest.fixture(scope="function")
+def event_loop(request):
+    request.param
+    loop = asyncio.new_event_loop()
+    yield loop
+    loop.close()
+
+
 @pytest.fixture(scope="function")
 def successful_queue():
     return SuccessfulQueue()