tesh kill timeouted processes with KILL also
It seems that the sanitizers processes are surviving their timeouts on
the server, so this may help fixing the build robots.
On the way, I switched to save the PID of the forked process instead
of its PGID. This helps when I try to see if the process is still
alive, later on: I can always do PID->PGID later (as I do now) while
it seems impossible to do PGID->PID to see whether the process is
still alive.
Fun fact: I think that PID=PGID on POSIX systems, but let's play safe :)
