test: fix test-http-bad-server with current python 2.7

Authored by jcristau.

Description

test: fix test-http-bad-server with current python 2.7

https://github.com/python/cpython/pull/2825 changed the exception
message for empty http status line.

Differential Revision: https://phab.mercurial-scm.org/D5412

Details