Allow the pythonpath to be adjusted for the client test module.
Allow the pythonpath to be adjusted for the client test module.

We added the ability to specify the client test module in PR #1254.  In
that PR, we started passing whatever the PYTHONPATH environment variable
was to the test.  Although this allows *one* test to override the path,
it fails when there are two such tests in different directories.  This
allows an additional parameter (TEST_PYTHONPATH) to be added to aid in
overriding the PYTHONPATH.  In this way, two tests that require
different paths can be run in the same build.

data-exp-lab/girder,Kitware/girder,Xarthisius/girder,manthey/girder,kotfic/girder,manthey/girder,jbeezley/girder,jbeezley/girder,RafaelPalomar/girder,Xarthisius/girder,girder/girder,data-exp-lab/girder,jbeezley/girder,girder/girder,girder/girder,Xarthisius/girder,manthey/girder,jbeezley/girder,RafaelPalomar/girder,RafaelPalomar/girder,Xarthisius/girder,kotfic/girder,data-exp-lab/girder,RafaelPalomar/girder,data-exp-lab/girder,data-exp-lab/girder,manthey/girder,girder/girder,kotfic/girder,Kitware/girder,kotfic/girder,Kitware/girder,RafaelPalomar/girder,Kitware/girder,Xarthisius/girder,kotfic/girder