Integration tests for pty.spawn on Linux and all other platforms

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2016-12-25.16:43:45.078>
labels = ['3.7', 'type-feature', 'tests']
title = 'Integration tests for pty.spawn on Linux and all other platforms'
updated_at = <Date 2020-08-09.22:13:49.544>
user = 'https://github.com/diekmann'

bugs.python.org fields:

activity = <Date 2020-08-09.22:13:49.544>
actor = 'soumendra'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Tests']
creation = <Date 2016-12-25.16:43:45.078>
creator = 'Cornelius Diekmann'
dependencies = []
files = ['46035', '46115', '46177', '46226', '46613', '46804']
hgrepos = []
issue_num = 29070
keywords = ['patch']
message_count = 12.0
messages = ['284003', '284492', '284669', '284670', '284842', '284942', '285038', '286471', '286683', '287450', '291713', '375085']
nosy_count = 6.0
nosy_names = ['xdegaye', 'martin.panter', 'Alex.Willmer', 'chris.torek', 'Cornelius Diekmann', 'soumendra']
pr_nums = ['2932']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue29070'
versions = ['Python 2.7', 'Python 3.5', 'Python 3.6', 'Python 3.7']