Wait for majority commit point to be advanced on each node before starting rollback test in rollback_set_fcv.js

We should call awaitLastOpCommitted after setting the FCV but before starting the rollback test.

This is because after setting the FCV, the secondary might not have updated its knowledge of the majority commit point through heartbeats before it is isolated from the set, so its lastFCVUpdateTimestamp will be after the majority commit point, which will cause checking the FCV to hang