test(progress): tighten regression assertion to assert_called_once

Address review feedback on PR #1554: update_idletasks() is called
exactly once per update_progress_bar() invocation, so the regression
test should assert exactly that. Matches the assertion style used
elsewhere in this test module and prevents a future change from
silently calling update_idletasks() multiple times per update.

Signed-off-by: Yash Goel <yashvardhan664@gmail.com>

Author: yashhzd

tests/test_frontend_tkinter_progress_window.py

@@ -278,7 +278,7 @@ def test_progress_updates_do_not_pump_user_events(self, progress_window) -> None
         progress_window.update_progress_bar(25, 100)
 
         progress_window.progress_bar.update.assert_not_called()
-        progress_window.progress_bar.update_idletasks.assert_called()
+        progress_window.progress_bar.update_idletasks.assert_called_once()
 
     def test_user_sees_progress_window_handle_lazy_window_relift(self, progress_window) -> None:
         """