Disable Flaky test test_operator.test_clip (apache#12902)

Author: marcoabreu

tests/python/unittest/test_operator.py

@@ -3618,6 +3618,7 @@ def test_special_functions_using_scipy():
 
 
 @with_seed()
+@unittest.skip("Flaky test, tracked at https://github.com/apache/incubator-mxnet/issues/12901")
 def test_clip():
     data = mx.symbol.Variable('data')
     shape = (30, 30)