Improve test coverage for idlelib

GitHub fields:

assignee = 'https://github.com/terryjreedy'
closed_at = None
created_at = <Date 2019-01-05.21:44:20.194>
labels = ['3.7', '3.8', 'expert-IDLE', 'type-feature', 'library']
title = 'Improve test coverage for idlelib'
updated_at = <Date 2019-05-09.04:54:19.385>
user = 'https://github.com/tonybaloney'

bugs.python.org fields:

activity = <Date 2019-05-09.04:54:19.385>
actor = 'anthonypjshaw'
assignee = 'terry.reedy'
closed = False
closed_date = None
closer = None
components = ['IDLE', 'Library (Lib)']
creation = <Date 2019-01-05.21:44:20.194>
creator = 'anthony shaw'
dependencies = []
files = []
hgrepos = []
issue_num = 35668
keywords = ['patch', 'patch', 'patch', 'patch']
message_count = 10.0
messages = ['333077', '333083', '333084', '333085', '333086', '333089', '333113', '333124', '333138', '333269']
nosy_count = 5.0
nosy_names = ['terry.reedy', 'taleinat', 'cheryl.sabella', 'anthonypjshaw', 'anthony shaw']
pr_nums = ['11451', '11451']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue35668'
versions = ['Python 3.7', 'Python 3.8']