Previously we only managed to verify a signature in case the certificate
was already imported in the local NSS db. Don't depend on that by
(temporarily) importing certificates from the PDF signature.

Also adjust a test file that failed previously (the test DB has only an
"Alice" cert imported, intentionally sign the file as "Bob" as well).

Change-Id: Id8440acc31915f5a1718ea48129b950bb67e7486