bpo-37707: Exclude expensive unit tests from PGO task (GH-15009)

Mark some individual tests to skip when --pgo is used.  The tests
marked increase the PGO task time significantly and likely don't
help improve optimization of the final executable.

Lib/test/test_itertools.py

@@ -2062,6 +2062,7 @@ class RegressionTests(unittest.TestCase):
         self.assertRaises(AssertionError, list, cycle(gen1()))
         self.assertEqual(hist, [0,1])
 
+    @support.skip_if_pgo_task
     def test_long_chain_of_empty_iterables(self):
         # Make sure itertools.chain doesn't run into recursion limits when
         # dealing with long chains of empty iterables. Even with a high