Improve bigmem tests

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2017-10-04.19:22:19.577>
labels = ['type-feature', 'tests']
title = 'Improve bigmem tests'
updated_at = <Date 2017-10-07.15:45:09.628>
user = 'https://github.com/serhiy-storchaka'

bugs.python.org fields:

activity = <Date 2017-10-07.15:45:09.628>
actor = 'pitrou'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Tests']
creation = <Date 2017-10-04.19:22:19.577>
creator = 'serhiy.storchaka'
dependencies = []
files = []
hgrepos = []
issue_num = 31695
keywords = []
message_count = 2.0
messages = ['303729', '303885']
nosy_count = 3.0
nosy_names = ['pitrou', 'vstinner', 'serhiy.storchaka']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue31695'
versions = []