bpo-20891: Skip test_embed.test_bpo20891() (#4967)

Skip the test failing randomly because of known race condition.

Skip the test to fix macOS buildbots until a decision is made on the
proper fix for the race condition.

Lib/test/test_embed.py

@@ -198,6 +198,9 @@ class EmbeddingTests(unittest.TestCase):
         self.assertEqual(out, '')
         self.assertEqual(err, '')
 
+    @unittest.skipIf(True,
+                     "FIXME: test fails randomly because of a race conditon, "
+                     "see bpo-20891")
     def test_bpo20891(self):
         """
         bpo-20891: Calling PyGILState_Ensure in a non-Python thread before