Use more valid data in TestPRSignedByIsSignatureAuthorAccepted
Use more valid data in TestPRSignedByIsSignatureAuthorAccepted

... to make sure we are exercising exactly the failure pathss we are
trying to test.

Signed-off-by: Miloslav Trmač <9c008e38982a5397deb855345fb164f0558459ae@redhat.com>

containers/image,containers/image