Here is an updated patch.  The changes I made are:

(1) Update code comments in _call_popen_and_assert().
(2) Fix test failure.
(3) Rename _call_popen_and_assert() to _assert_cwd() since it is a simpler
    name and the old name did not reflect that the method is specific to
    the test_cwd_* methods.
(4) Add _split_python_path() helper method so that we do not need to call
    os.path.realpath(sys.executable) in every method.

Andrew, you can make changes re: Tru64 (removing old code, etc).  Thanks.