Certificate storage tests

[touilleMan]

Tests to implement:

certificate already in database, new certificate is inserted which has same signature (e.g. same user_id for a UserCertificate), this should be checked:

• with and without the certificate already loaded in cache
• with the certificate in database invalid (cannot decrypt or cannot deserialize)
• with the certificate in database valid and the new certificate similar
• with the certificate in database valid and the new certificate different

~test get_last_certificate_timestamp with different certificates types, and with different dates !~ EDIT: no longer exist

[mmmarcos]

According to @TimeEngineer only the point regarding the cache is missing.

According to @touilleMan this is no longer a priority --> I am moving it out of the v3.0 milestone.