Larger and/or configurable _MAX_LENGTH for unittest messages

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2017-07-03.17:59:26.581>
labels = ['3.7', 'type-feature', 'library']
title = 'Larger and/or configurable _MAX_LENGTH for unittest messages'
updated_at = <Date 2017-07-05.20:27:37.208>
user = 'https://bugs.python.org/maarten-treewalker'

bugs.python.org fields:

activity = <Date 2017-07-05.20:27:37.208>
actor = 'r.david.murray'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2017-07-03.17:59:26.581>
creator = 'maarten-treewalker'
dependencies = ['27432']
files = []
hgrepos = []
issue_num = 30839
keywords = []
message_count = 2.0
messages = ['297605', '297779']
nosy_count = 2.0
nosy_names = ['r.david.murray', 'maarten-treewalker']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue30839'
versions = ['Python 3.7']