Add a test for get_id() (GH-21370)

diff --git a/Lib/test/test_symtable.py b/Lib/test/test_symtable.py
index d193558..fa51491 100644
--- a/Lib/test/test_symtable.py
+++ b/Lib/test/test_symtable.py
@@ -63,6 +63,13 @@
         self.assertEqual(self.spam.get_type(), "function")
         self.assertEqual(self.internal.get_type(), "function")
 
+    def test_id(self):
+        self.assertGreater(self.top.get_id(), 0)
+        self.assertGreater(self.Mine.get_id(), 0)
+        self.assertGreater(self.a_method.get_id(), 0)
+        self.assertGreater(self.spam.get_id(), 0)
+        self.assertGreater(self.internal.get_id(), 0)
+
     def test_optimized(self):
         self.assertFalse(self.top.is_optimized())