Don't override existing PYTHONPATH for tests.
Don't override existing PYTHONPATH for tests.

We can prepend some new paths at the beginning, but we don't want to
lose any existing module search paths when doing so.

krischer/python-future,QuLogic/python-future,QuLogic/python-future,PythonCharmers/python-future,michaelpacer/python-future,michaelpacer/python-future,krischer/python-future,PythonCharmers/python-future