Really fix with the zombie test (hopefully)

So, instead of any timeouts for synchronization, we move to actual
events. We still have one (huge) timeout for pathological cases (just to
cleanup), but we don't sleep at all in normal usage.

The patch moves the zombie/cleaned up child creation out from setUp(),
and to on-demand by the functions. We remove all signal handler usage,
instead we rely on the fact that when a process is cleaned up, its FDs
are closed, so the read end of a pipe will get an event.

Reviewed-by: imsnah