spec/test: add test units for Certificate handling in the votekeeper

[cason]

The ability of overwritten recorded votes, in the case of equivocation (see scenarios in this discussion), has lead to the design decision of producing and exchanging Prevote certificates as part of VoteSync protocol (#576, see this comment in particular).

The handling of certificates in the votekeeper was added to the Quint spec (#364) but the associated votekeeper tests were not updated accordingly. Since these tests are used as input for MBT-generated code tests, the implementation is not aligned with the model.

Definition of Done

• Add tests to the votekeeper unit that handle certificates
• In particular, make sure that votes in a certificate overwrite existing equivocating votes from the same process
• Add the produced tests to the MBT workflow, so that they are reproduced in the implementation