IDLE macOS: add tests

GitHub fields:

assignee = 'https://github.com/terryjreedy'
closed_at = None
created_at = <Date 2018-06-26.03:50:35.638>
labels = ['3.8', 'expert-IDLE', 'type-feature', '3.7']
title = 'IDLE macosx: add tests.'
updated_at = <Date 2018-12-12.02:34:48.495>
user = 'https://github.com/terryjreedy'

bugs.python.org fields:

activity = <Date 2018-12-12.02:34:48.495>
actor = 'terry.reedy'
assignee = 'terry.reedy'
closed = False
closed_date = None
closer = None
components = ['IDLE']
creation = <Date 2018-06-26.03:50:35.638>
creator = 'terry.reedy'
dependencies = []
files = []
hgrepos = []
issue_num = 33963
keywords = []
message_count = 4.0
messages = ['320468', '320469', '320471', '320671']
nosy_count = 3.0
nosy_names = ['terry.reedy', 'ned.deily', 'markroseman']
pr_nums = []
priority = 'normal'
resolution = None
stage = 'needs patch'
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue33963'
versions = ['Python 3.7', 'Python 3.8']