patcht/t3800-mktag.sh: use test_must_fail rather than '!'

diff --git a/t/t3800-mktag.sh b/t/t3800-mktag.sh
index 2780758..bdc6e13 100755
--- a/t/t3800-mktag.sh
+++ b/t/t3800-mktag.sh
@@ -14,7 +14,7 @@ test_description='git-mktag: tag object verify test'
 check_verify_failure () {
 	expect="$2"
 	test_expect_success "$1" '
-		( ! git-mktag <tag.sig 2>message ) &&
+		( test_must_fail git-mktag <tag.sig 2>message ) &&
 		grep "$expect" message
 	'
 }