Windows test fix: enhance the TestSCons class so an explicit

Windows test fix:  enhance the TestSCons class so an explicit
python=None argument still defaults to sys.executable.  Use this to
reset the python value we interpolate into the expected files that we
generate after resetting the os.environ['PYTHON'] value.

git-svn-id: http://scons.tigris.org/svn/scons/trunk@3445 fdb21ef1-2011-0410-befe-b5e4ea1792b1