bpo-42721: Improve using simple dialogs without root window (GH-23897)

When simple query dialogs (tkinter.simpledialog), message boxes
(tkinter.messagebox) or color choose dialog (tkinter.colorchooser)
are created without arguments master and parent, and the default
root window is not yet created, a new temporary hidden root window
will be created automatically. It will not be set as the default root
window and will be destroyed right after closing the dialog window.
It will help to use these simple dialog windows in programs which do
not need other GUI.

Previously, message boxes and color chooser created the blank root
window and left it after closing the dialog window, and query dialogs
just raised an exception.

Co-authored-by: Terry Jan Reedy <tjreedy@udel.edu>

Lib/tkinter/test/test_tkinter/test_messagebox.py

@@ -0,0 +1,38 @@
+import unittest
+import tkinter
+from test.support import requires, run_unittest, swap_attr
+from tkinter.test.support import AbstractDefaultRootTest
+from tkinter.commondialog import Dialog
+from tkinter.messagebox import showinfo
+
+requires('gui')
+
+
+class DefaultRootTest(AbstractDefaultRootTest, unittest.TestCase):
+
+    def test_showinfo(self):
+        def test_callback(dialog, master):
+            nonlocal ismapped
+            master.update()
+            ismapped = master.winfo_ismapped()
+            raise ZeroDivisionError
+
+        with swap_attr(Dialog, '_test_callback', test_callback):
+            ismapped = None
+            self.assertRaises(ZeroDivisionError, showinfo, "Spam", "Egg Information")
+            self.assertEqual(ismapped, False)
+
+            root = tkinter.Tk()
+            ismapped = None
+            self.assertRaises(ZeroDivisionError, showinfo, "Spam", "Egg Information")
+            self.assertEqual(ismapped, True)
+            root.destroy()
+
+            tkinter.NoDefaultRoot()
+            self.assertRaises(RuntimeError, showinfo, "Spam", "Egg Information")
+
+
+tests_gui = (DefaultRootTest,)
+
+if __name__ == "__main__":
+    run_unittest(*tests_gui)