After the changes on SERVER-54345 we need to wait for a write to reach majority when creating the shard index before continuing, to prevent that a rollback ends up having a sharded collection without the shard key index on the primary shard. However, the ephemeralForTest engine does not implement getLatestOplogTimestamp, causing a consistent failure when running any sharded test when using that engine. We could use something like waitForWriteConcern to wait for majority.