This issue follows-up on #27732, which suppressed beeps during IDLE tests, I want to also suppress the flashing of tk widget boxes, which has become visually obnoxious with the increasing number of tests.

Adding root.withdraw() after root = Tk() solves the problem for all current tests.  For test_textview, this requires the fix of a bad cleanup call and addition of another.  It appears that the textview toplevels must be destroyed before the cleanup call to root.update_idletasks() or else they are made visible.  The idletasks call is needed to avoid '''can't invoke "event" command''' (even with all toplevels gone).

The backports will have fewer changes because there are fewer tests.