Fixup flaky OC test

[carllin]

Problem

Removing the tower caused the validator to panic when restarting and revoting on a slot they had already voted on before the restart

Summary of Changes

1. Restart validator with no-voting flag, wait for him to make his leader slot X on the parent of the OC slot that we want to roll back, check it lands in ledger
2. Restart validator with the voting flag
3. Because the slot X is new, he won't have voted on it, so will avoid the gossip panic

Fixes #4060