Fix the tests of runtest.py now that QMTest is no longer being used

Fix the tests of runtest.py now that QMTest is no longer being used
by default.  Fix use of subprocess in Python 2.4+ and exit statuses
of popen'ed scripts in earlier versions of Python.  Support the ability
to execute a directory's tests by naming the directory as a command-
line argument.

git-svn-id: http://scons.tigris.org/svn/scons/trunk@4554 fdb21ef1-2011-0410-befe-b5e4ea1792b1