Runner: Close event loop in Runner.close()

This prevents leaving open event loops which cause `ResourceWarnings` to
be emitted during tests.

src/aiida/engine/runners.py

@@ -162,6 +162,8 @@ def close(self) -> None:
         """Close the runner by stopping the loop."""
         assert not self._closed
         self.stop()
+        if not self._loop.is_running():
+            self._loop.close()
         reset_event_loop_policy()
         self._closed = True