t5405: use test_must_fail() instead of checking exit code manually

This test expects "git push" to fail, thus it manually inverts that
local expected failure into a successful exit code for the test overall.
In doing so, it intentionally breaks the &&-chain. Modernize by
replacing manual exit code management with test_must_fail() and a normal
&&-chain.

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t5405-send-pack-rewind.sh

@@ -25,8 +25,7 @@ test_expect_success 'non forced push should die not segfault' '
 
 	(
 		cd another &&
-		git push .. master:master
-		test $? = 1
+		test_must_fail git push .. master:master
 	)
 
 '