Move responsibility for extracting the exit status from executed tests

Move responsibility for extracting the exit status from executed tests
from the TestCommon.py module to the wrapper classes in TestCmd.py
that we use as fallbacks if the subprocess module doesn't exist.

git-svn-id: http://scons.tigris.org/svn/scons/trunk@3751 fdb21ef1-2011-0410-befe-b5e4ea1792b1