[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).
raise NotImplementedError("Cannot migrate such an old version. Use intermediate Cubiweb version (try 3.16.x)")