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/conftest.py

@@ -162,6 +162,11 @@ def call_ray_start(request):
     # Kill the Ray cluster.
     subprocess.check_output(["ray", "stop"])
 
+@pytest.fixture
+def call_ray_stop_only():
+    yield
+    subprocess.check_output(["ray", "stop"])
+
 
 @pytest.fixture()
 def two_node_cluster():