ephemeralForTest cursors are not thread_safe (SERVER-42213). So fillWriterVectors could race with oplog insertions when trying to readTransactionOperationsFromOplogChain. We switched idempotency tests to use WiredTiger in SERVER-45288. I think we may need to do the same for all OplogApplierImplTest in general.

We should be careful with oplog visibility issues though after switching to WiredTiger. SERVER-42925 fixed oplog visiblity issues in idempotency tests by introducing an oplogDiskLocRegister call to update the oplog visibility timestamp after applying each batch. But in non-idempotency tests like this one, we call applyOplogBatch directly which I believe doesn't update the oplog visibility timestamp.