t0008: use named pipe (FIFO) to test check-ignore streaming
sleeps in the check-ignore test suite are not ideal since they can
fail when the system is under load, or when a tool like valgrind is
used which drastically alters the timing. Therefore we replace them
with a more robust solution using a named pipe (FIFO).
Thanks to Jeff King for coming up with the redirection wizardry
required to make this work.