[test] Add user-site to PYTHONPATH before installation of new cube in test_newcube_install Otherwise, setuptools complains that the target installation directory (i.e. "user site" is not in PYTHONPATH). This is probably because we use a fake HOME with --user install option. Failure shows up when running tests through tox without system site packages (see next patch).

Twisted < 16.0.0
requests
webtest
cubicweb-blog
cubicweb-file
cubicweb-tag