Use server fixture for network test

[chrahunt]

Working through tests one at a time to help figure out the test server interface. This was one of our longest tests, 13 seconds before and 2 afterwards on my machine.

Progresses #259.

[xavfernandez]

I think it would help future test debugging to add a (maybe optional) expected_path_info="/simple1/initools/" arg to package_page (and other fake pages).

pip/tests/lib/server.py

Lines 122 to 127 in 9ae2e64

	def responder(environ, start_response):
	# type: (Environ, StartResponse) -> Body
	start_response("200 OK", [
	("Content-Type", "text/html; charset=UTF-8"),
	])
	return [text.encode('utf-8')]

would return a 500 - Unexpected path_info: /foo/bar while expecting /simple1/initools/.

[chrahunt]

It seems like it might be easy to get lost in the possible error states - pip would've already done the wrong thing and then it will do more (unexpectedly) in response to the server error.