Add split install targets to install tests separately from lib

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2021-02-21.04:49:05.806>
labels = ['type-feature', '3.10', 'build']
title = 'Add split install targets to install tests separately from lib'
updated_at = <Date 2021-04-11.22:17:47.499>
user = 'https://github.com/eli-schwartz'

bugs.python.org fields:

activity = <Date 2021-04-11.22:17:47.499>
actor = 'eschwartz'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Build']
creation = <Date 2021-02-21.04:49:05.806>
creator = 'eschwartz'
dependencies = []
files = []
hgrepos = []
issue_num = 43282
keywords = []
message_count = 3.0
messages = ['387438', '387453', '390801']
nosy_count = 2.0
nosy_names = ['vstinner', 'eschwartz']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue43282'
versions = ['Python 3.10']