This allows:
- Running scripts via shebang (not important here)
- Progress output (so we can ditch more of our run script)
- Context (only after, for now) - this is important if there is a test failure
Closes#6435.
close_fds=True is actually the default in Python 2.7 and 3.2, but not in
ancient (but still in production in Red Hat Enterprise Linux 6) Python
2.6. Enable it there as well.
