Fix redis-server stoping in linux (#6296)

* Cleanup test_calling_start_ray_head

* Kill redis-server with args instead of comm

In linux, ps -o pid,comm output just redis-server instead of the
full executable path

python/ray/tests/test_multi_node.py

@@ -321,7 +321,7 @@ print("success")
         process_handle.kill()
 
 
-def test_calling_start_ray_head():
+def test_calling_start_ray_head(call_ray_stop_only):
     # Test that we can call ray start with various command line
     # parameters. TODO(rkn): This test only tests the --head code path. We
     # should also test the non-head node code path.