tests: factor portable signal check out of t0005

In POSIX shells, a program which exits due to a signal
generally has an exit code of 128 plus the signal number.
However, ksh uses 256 plus the signal number.  We've
accounted for that in t0005, but not in other tests.  Let's
pull out the logic so we can use it elsewhere.

It would be nice for debugging if this additionally printed
errors to stderr, like our other test_* helpers. But we're
going to need to use it in other places besides the innards
of a test_expect block. So let's leave it as generic as
possible.

Note that we also leave the magic "3" for Windows out of the
generic helper. This is an artifact of the way we use
raise() to kill ourselves in test-sigchain.c, and will not
necessarily apply to all programs. So it's better to keep it
out of the helper, to reduce the chance of confusing it with
a real call to exit(3).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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