IDLE: Improve configuration tests with mock Save.

GitHub fields:

assignee = 'https://github.com/terryjreedy'
closed_at = None
created_at = <Date 2017-07-07.02:56:35.781>
labels = ['expert-IDLE', 'type-feature', '3.7']
title = 'IDLE: Improve configuration tests with mock Save.'
updated_at = <Date 2017-07-09.11:06:22.147>
user = 'https://github.com/terryjreedy'

bugs.python.org fields:

activity = <Date 2017-07-09.11:06:22.147>
actor = 'cheryl.sabella'
assignee = 'terry.reedy'
closed = False
closed_date = None
closer = None
components = ['IDLE']
creation = <Date 2017-07-07.02:56:35.781>
creator = 'terry.reedy'
dependencies = []
files = []
hgrepos = []
issue_num = 30868
keywords = []
message_count = 5.0
messages = ['297854', '297970', '297975', '297983', '297990']
nosy_count = 2.0
nosy_names = ['terry.reedy', 'cheryl.sabella']
pr_nums = []
priority = 'normal'
resolution = None
stage = 'needs patch'
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue30868'
versions = ['Python 3.6', 'Python 3.7']