t7510-signed-commit: use 'test_must_fail'

The two tests 'detect fudged signature' and 'detect fudged signature
with NUL' in 't7510-signed-commit.sh' check that 'git verify-commit'
errors out when encountering a forged commit, but they do so by
running

  ! git verify-commit ...

Use 'test_must_fail' instead, because that would catch potential
unexpected errors like a segfault as well.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t7510-signed-commit.sh

@@ -145,7 +145,7 @@ test_expect_success GPG 'detect fudged signature' '
 
 	sed -e "s/seventh/7th forged/" raw >forged1 &&
 	git hash-object -w -t commit forged1 >forged1.commit &&
-	! git verify-commit $(cat forged1.commit) &&
+	test_must_fail git verify-commit $(cat forged1.commit) &&
 	git show --pretty=short --show-signature $(cat forged1.commit) >actual1 &&
 	grep "BAD signature from" actual1 &&
 	! grep "Good signature from" actual1
@@ -156,7 +156,7 @@ test_expect_success GPG 'detect fudged signature with NUL' '
 	cat raw >forged2 &&
 	echo Qwik | tr "Q" "\000" >>forged2 &&
 	git hash-object -w -t commit forged2 >forged2.commit &&
-	! git verify-commit $(cat forged2.commit) &&
+	test_must_fail git verify-commit $(cat forged2.commit) &&
 	git show --pretty=short --show-signature $(cat forged2.commit) >actual2 &&
 	grep "BAD signature from" actual2 &&
 	! grep "Good signature from" actual2