tests: sign-verify.sh: Add test for no trusted certificate

If there's no CA then sbverify should always fail (having no
certificate to chain the signature to).  This was erroneously passing
up to commit db731f0 ("sbverify: fix intermediate certificate
verification") so add this test to ensure no regression.

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