[PATCH 3 of 3 v2 pip-fix] tests: add a test for installing hg with pip in a virtualenv

[Danek Duvall]

I've been running into problems with this.  It's not a Solaris issue, but
because I'm running inside a non-transparent firewall.

http_proxy is being stripped from the environment of each test, but in
order to do a pip install, I need that environment variable in order to
connect outside.

I also need to have virtualenv on the system, so I can't disable the test
by making sure the #if evaluates to false.

I'm not sure why this isn't failing on the buildbot instances, which should
have the same issue.

Any thoughts on what to do here?  The ping command has a different
interface on Solaris than on Linux, so trying to simply ping
pypi.python.org (assuming we can rely on pip always trying that host) is
going to run into platform-specific issues.  We could save http_proxy
aside, and enable it here specifically (somehow).  Or we could allow for
specific "have"s to be blacklisted, and introduce some sort of
external-network #if token, maybe?

Danek