Fix tox tests in an sbuild chroot

Currently the tests are disabled in debian/rules. This is because the test suite uses tox, and by default tox puts its virtualenvs inside of $HOME/.tox. In an sbuild $HOME is set to /sbuild-nonexistent so tox can't even set things up.

I actually think this is counter to tox's own documentation, which states that:

[tox]
minversion=ver # minimally required tox version
toxworkdir=path # tox working directory, defaults to {toxinidir}/.tox
setupdir=path # defaults to {toxinidir}
distdir=path # defaults to {toxworkdir}/dist
distshare=path # defaults to {homedir}/.tox/distshare
envlist=ENVLIST # defaults to the list of all environments

Notice that toxworkdir says {toxinidir}/.tox and of course in an sbuild the directory containing tox.ini is /root/package-blah/package_blah so this *should* work.

The options are:

 * See if tox 1.5 fixes this
 * Try to set a not unreasonable $HOME for the test invocation
 * Don't run the tests
 * Create a tox.ini environment setting toxinidir to something reasonable
 * Run the tests differently in sbuild (click-package does this - it uses tox but also plain unittest in the setup.py, probably specifically for this reason).