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Our "make clean" detection was causing problems for tests which had
their own local clean target.

We now run programs in a Job, which means that we can kill a process
and all of its children when a timeout happens.

A better timeout.  This one starts a new session for the child
process, and attempts to kill the entire group when the time expires
(previously we only killed the direct child, if the child itself had
spawned more processes these would continue to run).

The new scheme is only for Unix, presumably we have to do something
different on Windows.

Code partly from Ian Lynagh.

Add a timeout to test runs, using a wrapper program (written in
Haskell, using System.Process of course!).