rollback_after_enabling_majority_reads.js test should ensure the correct primary before restarting other nodes [v4.0]

The rollback_after_enabling_majority_reads.js test expects node 0 to become primary here, by shutting down and restarting node 1. This may not always work if node 1 restarts and wins an election too soon. We should make this test wait for node 0 to become primary in this case before node 1 is restarted or is allowed to get elected.