The intent of the test, introduced in SERVER-30212, is to verify multiple uncommitted WriteUnitOfWork that have oplog optimes assigned by repl::logOp() and not to stress the ephemeralForTest storage engine's support for concurrent document insert operations on the oplog collection. By synchronizing access to repl::logOp() with the mutex that's already present in the tests, we can still preserve the existing test cases in oplog_test.cpp which illustrate how logOp() works with storage engines that support document level concurrency.