Currently the ServiceContextMongoDTest is the only fixture which instantiates storage engine and many tests depend on that. However it can only create an ephemeralForTest storage engine, which makes it unsuitable for unit-tests, which need to exercise concurrency.

It should be extended with a constructor, which accepts StorageGlobalParams in order to allow it to be configured with a storage engine:

ServiceContextMongoDTest(const StorageGlobalParams& storageParams);