TST: Ignore nan-warnings in randomized out tests

The tests randomize the nan pattern and thus can run into these
(additional) warnings, so ignore them.
(Could also fix the random seed, but this should do)

Closes gh-22835

numpy/lib/tests/test_nanfunctions.py

@@ -818,6 +818,7 @@ def test_keepdims(self):
             (-3, -1),
         ]
     )
+    @pytest.mark.filterwarnings("ignore:All-NaN slice:RuntimeWarning")
     def test_keepdims_out(self, axis):
         d = np.ones((3, 5, 7, 11))
         # Randomly set some elements to NaN:
@@ -1021,6 +1022,7 @@ def test_keepdims(self):
             (-3, -1),
         ]
     )
+    @pytest.mark.filterwarnings("ignore:All-NaN slice:RuntimeWarning")
     def test_keepdims_out(self, q, axis):
         d = np.ones((3, 5, 7, 11))
         # Randomly set some elements to NaN: