[stacks-signer] DKG related testing scenarios

[jferrant]

There are a few DKG scenarios that do not seem to be properly tested:

• If DKG fails due to a timeout (i.e. non responsive signers), there should be a new DKG round triggered that excludes those unresponsive signers
• If coordinator changes mid DKG round, the round should be able to complete with the new coordinator. ---> signers with a different view of the coordinator should not stall progress for the SAME round id.