parallelize test suite

[David Bremner]

"Rollins, Jameson" <jrollins at caltech.edu> writes:

> This is a simple patch series that will run the entire test suite in
> parallel if either the moreutils or GNU parallel utility is
> available.  On my 8-core machine the full test suite will now run in
> under 20 seconds, which is a pretty huge improvement.

I've taken the plunge and merged this series to master. I also posted an
update to smtp-dummy following Tomi's suggestion to bind to port 0.

d