Handle unclean connection closures in close()

Also fixes the unit tests that uses mock connection

Author: cderici

juju/client/connection.py

@@ -456,6 +456,9 @@ async def close(self, to_reconnect=False):
             await jasyncio.gather(*jasyncio.all_tasks())
         except jasyncio.CancelledError:
             pass
+        except websockets.exceptions.ConnectionClosed:
+            pass
+
         self._pinger_task = None
         self._receiver_task = None
         self._debug_log_task = None