t7510-signed-commit: use 'test_must_fail'

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>