Fix bug in check-webkit-style resulting from our PYTHONPATH shenanigans.

pylint was getting confused by our use of unittest2 renamed as unittest; we
need to have Tools/Scripts and Tools/Scripts/webkitpy/thirdparty in sys.path
for this to work correctly, so we also need to export it into PYTHONPATH.
Without this we would occassional get false errors about missing members
in the TestCase classes.

R=ojan@chromium.org
BUG=

Review URL: https://codereview.chromium.org/23756004

git-svn-id: svn://svn.chromium.org/blink/trunk@157240 bbb929c8-8fbe-4397-9dbb-9b2b20218538