pydrake: Rename a unittest method to avoid determinism bug (RobotLoco…

…motion#11719)

This test case fails under python3 -c dbg with certain names, but not others.
This works around spurious failures that show up as of commit
12a8432.

bindings/pydrake/systems/test/custom_test.py

@@ -227,7 +227,7 @@ def test_leaf_system_per_item_tickets(self):
                 ]:
             self.assertIsInstance(func(arg), DependencyTicket, func)
 
-    def test_leaf_system_overrides(self):
+    def test_all_leaf_system_overrides(self):
         test = self
 
         class TrivialSystem(LeafSystem):