Added a test

ml-agents/tests/envs/test_rpc_communicator.py

     first_comm.close()
 
 
+def test_rpc_communicator_close():
+    first_comm = RpcCommunicator()
+    first_comm.close()
+    second_comm = RpcCommunicator()
+    second_comm.close()
+
+
 def test_rpc_communicator_create_multiple_workers():
     # Ensures multiple RPC communicators can be created with
     # different worker_ids without causing an error.