Fix no cpus test (#5009)

python/ray/tests/conftest.py

@@ -56,6 +56,14 @@ def _ray_start(**kwargs):
     ray.shutdown()
 
 
+# The following fixture will start ray with 0 cpu.
+@pytest.fixture
+def ray_start_no_cpu(request):
+    param = getattr(request, "param", {})
+    with _ray_start(num_cpus=0, **param) as res:
+        yield res
+
+
 # The following fixture will start ray with 1 cpu.
 @pytest.fixture
 def ray_start_regular(request):