Fix race condition in kill test leading to hang

When you open a FIFO for reading, but there's no writer, you hang.
This is just one of those obscure UNIXisms we all know but just
forget all too often.

My last PR was guilty of introducing such a condition; I caught
it by accident while testing other stuff. In short, the signal
container was doing 'echo DONE' as its last step, and we (BATS)
were reading the FIFO to check for it; but if the container
exited before we opened the FIFO for read, the open would hang.
This is not a hang that we can catch in the test: it would hang
the entire job forever. CI would presumably time out eventually,
but with no useful indication of the cause of the error.

Solution: use 'exec' to open the FIFO early and keep it open,
and use 'read -u FD' instead of 'read <$fifo': the former
reads from an open FD, the latter forces a new open() each time.

There is a shorter, more maintainable solution -- see #4755 -- but
that suffers from the same hanging problem in the (unlikely) case
where the signal-handling container exits, e.g. if signal handling
is broken in podman. The test would hang, with no helpful indicator.
Although this PR is a little more advanced scripting, I have
commented the relevant code well and believe the maintenance
cost is worth the risk of undebuggable hangs.

There is still a hang risk: if 'podman logs -f' fails and exits
immediately, the 'exec' will hang. I can't think of a non-racy
way to prevent that, and choose to live with that risk.

Tested by temporarily including 9 (SIGKILL) in the signals list.
The read timeout triggers, and the end user has a fair chance
of tracking down the root cause.

Signed-off-by: Ed Santiago <santiago@redhat.com>

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