Currently we launch processes via the shell in a few places only to
redirect standard output and error to a log file ("&> $file"). It is
better to do such redirection from within RunCmd itself.

This patch splits RunCmd in two parts, the setup and the execution part,
the latter being implemented in two different functions depending on
whether we write to a file or not.

We also update the unittests with this new case.

Reviewed-by: imsnah