Apply clock changes to running nodes in the consensus tests

As part of https://github.com/input-output-hk/ouroboros-network/issues/759 (PR https://github.com/input-output-hk/ouroboros-network/pull/1554) we introduced tests for the `BlockchainTime` in the presence of an unstable clock. However, it would be good to extend also the consensus tests to apply clock changes, to catch stuff like https://github.com/input-output-hk/ouroboros-network/issues/1570 and https://github.com/input-output-hk/ouroboros-network/pull/1544#discussion_r374608184 .