t/lib-git-daemon: use test_match_signal

When git-daemon exits, we expect it to be with the SIGTERM
we just sent it. If we see anything else, we'll complain.
But our check against exit code "143" is not portable. For
example:

  $ ksh93 t5570-git-daemon.sh
  [...]
  error: git daemon exited with status: 271

We can fix this by using test_match_signal.

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

t/lib-git-daemon.sh

@@ -82,8 +82,7 @@ stop_git_daemon() {
 	kill "$GIT_DAEMON_PID"
 	wait "$GIT_DAEMON_PID" >&3 2>&4
 	ret=$?
-	# expect exit with status 143 = 128+15 for signal TERM=15
-	if test $ret -ne 143
+	if test_match_signal 15 $?
 	then
 		error "git daemon exited with status: $ret"
 	fi