test: fix helios zk failover test spurious failure

`ZooKeeperHeliosFailoverTest.verifyCanDeployWithOneNodeDeadAfterOneNodeDataLoss` frequently fails. This PR is an attempt to fix that.

The test was racy and could end up with just one single intact node, one node down and one node without any data. The ZK quorum would then be broken, and it's not fair to expect to be able to make progress beyond that point.