Run Python's slowest tests in the first 3/4 of tests when using -r

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2017-05-28.19:41:20.833>
labels = ['3.7', 'tests']
title = "Run Python's slowest tests in the first 3/4 of tests when using -r"
updated_at = <Date 2017-06-03.18:25:10.238>
user = 'https://github.com/brettcannon'

bugs.python.org fields:

activity = <Date 2017-06-03.18:25:10.238>
actor = 'brett.cannon'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Tests']
creation = <Date 2017-05-28.19:41:20.833>
creator = 'brett.cannon'
dependencies = []
files = []
hgrepos = []
issue_num = 30498
keywords = []
message_count = 5.0
messages = ['294658', '294660', '295061', '295090', '295095']
nosy_count = 3.0
nosy_names = ['brett.cannon', 'terry.reedy', 'serhiy.storchaka']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = None
url = 'https://bugs.python.org/issue30498'
versions = ['Python 3.7']