May depend on:

• feat(node/actor): Make SequencerActor generic over new tx gossip trait #3068
• feat(node/actor): Make SequencerActor generic over new unsafe head access trait #3069

Now that some SequencerActor dependencies are injected and easily mockable, this task is to add unit tests for SequencerActor functionality. Specific areas of focus are:

• Block building loop/timing
• Block building error handling (how all build/seal errors are handled)
• Block building race conditions
  • e.g. unsafe head changing between build and seal calls
  • other cases in which current state dictates that block building is not possible
• Admin API handling

It will likely be useful to create tooling to create mocks for dependencies. Some dependencies will ultimately be reused across actors, so this tooling could be useful in multiple places.