IDLE: add tests for help_about.py

GitHub fields:

assignee = 'https://github.com/terryjreedy'
closed_at = None
created_at = <Date 2017-05-06.05:03:19.798>
labels = ['expert-IDLE', 'type-feature', '3.7']
title = 'IDLE: add tests for help_about.py'
updated_at = <Date 2019-01-17.22:28:53.730>
user = 'https://github.com/terryjreedy'

bugs.python.org fields:

activity = <Date 2019-01-17.22:28:53.730>
actor = 'terry.reedy'
assignee = 'terry.reedy'
closed = False
closed_date = None
closer = None
components = ['IDLE']
creation = <Date 2017-05-06.05:03:19.798>
creator = 'terry.reedy'
dependencies = ['30303']
files = []
hgrepos = []
issue_num = 30290
keywords = []
message_count = 24.0
messages = ['293152', '293998', '294004', '294058', '294064', '294079', '294082', '294092', '294094', '294095', '294100', '294114', '294115', '294121', '294139', '294298', '294347', '294608', '294616', '294620', '294625', '295302', '295615', '333903']
nosy_count = 3.0
nosy_names = ['terry.reedy', 'louielu', 'cheryl.sabella']
pr_nums = ['1697', '1714', '1838', '2068', '2070']
priority = 'normal'
resolution = None
stage = 'commit review'
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue30290'
versions = ['Python 3.6', 'Python 3.7']