Race in read_concern_majority_getmore_secondaries.js

The test stops replication on two secondaries using the rsSyncApplyStop failpoint, then writes to the primary. It assumes that these writes will not be majority committed. However, it's possible for one or more secondaries to continue replicating and acknowledging writes until they notice the failpoint has been set. Let's use the waitForFailPoint command to ensure replication has really stopped before executing the writes that we don't want committed.