tests: close asyncio tasks before closing the loop

cylc/uiserver/conftest.py

@@ -0,0 +1,16 @@
+import asyncio
+
+import pytest
+
+
+@pytest.fixture
+def event_loop():
+    """This fixture defines the event loop used for each test."""
+    loop = asyncio.get_event_loop_policy().new_event_loop()
+    yield loop
+    # gracefully exit async generators
+    loop.run_until_complete(loop.shutdown_asyncgens())
+    # cancel any tasks still running in this event loop
+    for task in asyncio.all_tasks(loop):
+        task.cancel()
+    loop.close()