bpo-33855: More edits and new minimal tests for IDLE (GH-7761)

Part 2 of 3.  Continues PR #7689, changeset ee5ef30.
Edit and add tests for 18 modules, help_about to replace and run.

Lib/idlelib/idle_test/test_mainmenu.py

@@ -0,0 +1,21 @@
+"Test mainmenu, coverage 100%."
+# Reported as 88%; mocking turtledemo absence would have no point.
+
+from idlelib import mainmenu
+import unittest
+
+
+class MainMenuTest(unittest.TestCase):
+
+    def test_menudefs(self):
+        actual = [item[0] for item in mainmenu.menudefs]
+        expect = ['file', 'edit', 'format', 'run', 'shell',
+                  'debug', 'options', 'windows', 'help']
+        self.assertEqual(actual, expect)
+
+    def test_default_keydefs(self):
+        self.assertGreaterEqual(len(mainmenu.default_keydefs), 50)
+
+
+if __name__ == '__main__':
+    unittest.main(verbosity=2)