IDLE: Make GUI test teardown less fragile

GitHub fields:

assignee = 'https://github.com/terryjreedy'
closed_at = None
created_at = <Date 2017-08-26.16:55:38.220>
labels = ['expert-IDLE', 'type-feature', '3.7']
title = 'IDLE: Make GUI test teardown less fragile'
updated_at = <Date 2017-08-27.20:10:56.780>
user = 'https://github.com/csabella'

bugs.python.org fields:

activity = <Date 2017-08-27.20:10:56.780>
actor = 'terry.reedy'
assignee = 'terry.reedy'
closed = False
closed_date = None
closer = None
components = ['IDLE']
creation = <Date 2017-08-26.16:55:38.220>
creator = 'cheryl.sabella'
dependencies = []
files = []
hgrepos = []
issue_num = 31284
keywords = []
message_count = 6.0
messages = ['300885', '300908', '300909', '300912', '300919', '300925']
nosy_count = 2.0
nosy_names = ['terry.reedy', 'cheryl.sabella']
pr_nums = ['3212']
priority = 'normal'
resolution = None
stage = 'needs patch'
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue31284'
versions = ['Python 3.6', 'Python 3.7']