gh-154357: Fix race condition in test_simpledialog by explicitly setting focus to entry

[arshalaromal]

Fixes gh-154357.

Summary of Changes

In Lib/test/test_tkinter/test_simpledialog.py, enter() was generating <Return> events directly on the top-level dialog d without ensuring d.entry had active focus. On headless or slow CI runners, this caused key events to get dropped, leaving d.result as None and leading to flaky failures in test_boundary_values_accepted.

This PR fixes the race condition by:

1. Explicitly focusing d.entry via d.entry.focus_set().
2. Flushing pending idle events with d.update_idletasks() before generating the key event directly on d.entry.

Cc @serhiy-storchaka

[serhiy-storchaka]

Sorry, but I am already working on more complete fix #154370.