I don't know anything about idlelib, but it seems like the following patch fixes the failing test:

diff --git a/Lib/idlelib/idle_test/test_configdialog.py b/Lib/idlelib/idle_test/test_configdialog.py
index aff3c2f..9f495fd 100644
--- a/Lib/idlelib/idle_test/test_configdialog.py
+++ b/Lib/idlelib/idle_test/test_configdialog.py
@@ -41,6 +41,7 @@ def tearDownModule():
     global root, dialog
     idleConf.userCfg = usercfg
     tracers.detach()
+    tracers.clear()
     del dialog
     root.update_idletasks()
     root.destroy()

Extract of ConfigDialog.create_page_highlight():

        self.builtin_theme = tracers.add(
                StringVar(parent), self.var_changed_builtin_theme)

This method contains many tracers.add() calls, but tearDownModule() of Lib/idlelib/idle_test/test_configdialog.py only calls "tracers.detach()" and detatch() doesn't clear VarTrace.untraced list.