Assert the two user-visible changes: SHA-256 hashes and the digest of
the signing certificate.

Change-Id: I0f931ef06f9bfc4be4eaa02a7530d57a414430c1
Reviewed-on: https://gerrit.libreoffice.org/31172Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Jenkins <ci@libreoffice.org>