Add check using policy_seasalp_with_witnesses

Add a check using a policy file called "policy_seasalp_with_witnesses". It is assumed that such a file has already been created on the server.

This is currently being tested was briefly tested (succeeded at first, then failed due to not enough cosignatures) on the tee server, related files can be seen in the /var/sigsumchecker/log_submission_test directory there.