bpo-33855: Still more edits and minimal tests for IDLE (GH-7784)

Part 3 of 3, continuing PR #7689. This covers 14 idlelib modules and their tests,
rpc to zoomheight except for run (already done) and tooltip (being done separately).

Lib/idlelib/tree.py

@@ -462,6 +462,8 @@ def _tree_widget(parent):  # htest #
     node.expand()
 
 if __name__ == '__main__':
-    # test_tree is currently a copy of this
+    from unittest import main
+    main('idlelib.idle_test.test_tree', verbosity=2, exit=False)
+
     from idlelib.idle_test.htest import run
     run(_tree_widget)