t0005: skip signal death exit code test on Windows

The test case depends on that test-sigchain can commit suicide by a
call to raise(SIGTERM) in a way that run-command.c::wait_or_whine()
can detect as death through a signal. There are no POSIX signals on
Windows, and a sufficiently close emulation is not available in the
Microsoft C runtime (and probably not even possible).

The particular deficiency is that when a signal is raise()d whose
SIG_DFL action will cause process death (SIGTERM in this case), the
implementation of raise() in msvcrt just calls exit(3).

We could check for exit code 3 in addition to 143, but that would
miss the point of the test entirely. Hence, just skip it on Windows.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Acked-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>