bpo-42142: Try to fix timeouts in ttk tests (GH-23474)


Instead of using wait_visibility() which waits event <VisibilityNotify> in dead loop
use update() which should proceed all queued events.
(cherry picked from commit 6cc2c419f6cf5ed336609ba01055e77d7c553e6d)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>
2 files changed