tests: conditionalize test-http-bad-server.t for Python 3.5

Authored by indygreg.

Description

tests: conditionalize test-http-bad-server.t for Python 3.5

It appears that Python 3 introduced output buffering in the HTTP
response stack. And Python 3.6 switched from sock.makefile().write()
to sock.sendall().

So, we need to conditionalize test-http-bad-server.t to account
for the difference in behavior between Python 3.5 and 3.6.

Differential Revision: https://phab.mercurial-scm.org/D5754