Integration test kill -9-ing a node

[ktoso]

Would be a good test.

Run some nodes, kill -9 one, see how the down propagates.

This mostly exercises:

• swim, so that it detects the problem
• swim and membership integration, that it bubbles up
• downing strategy impl, that it calls a down
• gossip, no weird "come back alive" may happen
• connection management, so that we don't keep around dead associations
• connection management in general, no weird coming back alive may happen

"timer downing" is good enough for this.

[ktoso]

Follow up to https://github.com/apple/swift-distributed-actors/pull/375