check_kernel_signature() in CKCT didn't fail as expected

[Po-Hsu Lin]

While debugging bug 1828769 for a s390x KVM, I found this check_kernel_signature() is not working properly.

This will check the kernel signature with "sbverify" command, but the package is not available on s390x.

However, this test didn't fail.

        (result, output) = s.ssh('sudo sbverify --cert /usr/share/grub/canonical-uefi-ca.crt "/boot/vmlinuz-$(uname -r)"', ignore_result=False)
        if result != 0:
            raise ErrorExit("The Kernel Signature Check for %s has Failed" % s.raw_target)