NetBSD's sleep quits when foregrounded sometimes. I'm not entirely
sure *why*, but this is reproducible with the default /bin/sh, so it's
not our fault.
Because this fails our tests, go back to using cat *there*, because we
can't use it on macOS - 4c9d01cab0.
Apparently on macOS SIGTSTP (from control-Z) causes `read()` to return
EINTR.
This means `cat | cat` will exit as soon as it's backgrounded and
brought back.
So instead we use `sleep`, which won't read(), and therefore is
impervious to these puny attacks.
See discussion in #7447.
