cf-upgrade should not close STDOUT of all forked commands

Now both STDOUT and STDERR of forked commands go to STDOUT and STDERR of
cf-upgrade. Previously it was just closed and e.g. dpkg was failing
because of that.

Modify the test scripts to actually test that.
Before this patch a bunch of files were being created, but they were always empty. The spawn command's stdout was always closed. It might have to do with the close-on-exec flag.

Furthermore having a bunch of files thrown out in the directory is ugly. So I keep all output to stdout/stderr. If possible I'd prefer to log those to the starting log file.