[3.13] gh-155648: Write the empty and placeholder IDLE tests (GH-156260)

[serhiy-storchaka]

Restore the placeholder import in idlelib/idle_test/template.py
It was removed as an unused import in GH-151478, but template.py is a
skeleton for creating new IDLE test files, and idle_test/README.txt
instructs the user to replace 'zzdummy' with the name of the module
under test. Add a Ruff per-file ignore to keep it.

test_editor.RMenuTest was added in GH-18951, which fixed right-clicking
inside a selection, with the note that an automated test should follow.
Use the DummyRMenu class left there to test right_menu_event(), and test
the rmenu_check_*() methods that supply the menu entry states.

test_configdialog.ConfigDialogTest was left with two empty stubs in
GH-3592, named after the two ConfigDialog methods which the button tests
only check to be called. Test them with a fake parent whose instance
dictionary contains an autospecced EditorWindow.

test_configdialog.ExtPageTest was added empty, with a commented-out
"Nothing here yet TODO" skip, when ExtPage was factored out of
ConfigDialog in GH-26618. Test load_extensions(), extension_selected(),
set_extension_value() and save_all_changed_extensions().

test_grep.Default_commandTest was left empty in 2013 because
GrepDialog.default_command() imports OutputWindow when called, and the
import cannot be moved to the top of the module due to an import loop.
Replace the imported class with a mock instead of moving the import.

test_config.ChangesTest.test_save_default never called save_all(), so
it tested nothing. Add the missing assertions, and add the test for
the Save() calls that the following TODO comment asked for.

test_config.IdleConfTest.test_get_current_keyset only tested the
non-darwin branch, because the default key sets no longer contain Alt
keys. Add an extension binding with an Alt key, so that its
replacement with Option can be tested. Remove the stale commented-out
test in test_get_extension_keys, which used the ZoomHeight extension.

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Co-authored-by: Claude Opus 5 (1M context) noreply@anthropic.com
Co-authored-by: Hugo van Kemenade 1324225+hugovk@users.noreply.github.com
Co-authored-by: TerryJReedy tjreedy@udel.edu

(cherry picked from commit f8f8c30)

Co-authored-by: Serhiy Storchaka storchaka@gmail.com

• Issue: IDLE: Fix devdanzin test issues #155648