tests: Add verification of signing with expired certificates

Update the test runners to understand the skip test exit code (77) so
the verify-expired.sh test can skip if the necessary faketime binary
isn't installed.  Create an expired cert and a CA which is valid from
before the expired cert then make sure openssl fails certificate
validation at the current time but that the sbverify still passes it
(so ignores the expiry date).

Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>