Update state-machine tests for the CertDB (and VoteDB) to test parallel behavior

[tbagrel1]

Since the CertDB (and VoteDB) will be used by multiple threads, we should test that there is no deadlock or concurrency issues happening. This require changing our state-model testing backend, or at least enabling the parallel test feature

Initially raised by IntersectMBO/ouroboros-consensus#1784 (comment)