Fix race condition bug in testing scenario framework ChainExpectStacksTenureChange::apply

[jferrant]

Given how disallow_reorg_within_first_proposal_burn_block_timing_secs_but_more_than_one_block_scenario is structured, you cannot currently gaurantee that the block has NOT already been processed by the node when calling get_peer_stacks_tip_height() causing a potential off by one race condition in ChainExpectStacksTenureChange::apply.

There is a similar race condition within ChainExpectNakaBlock