Whenever a ServiceContextMongoDTest fixture is constructed, it initializes a new storage engine. This storage engine initialization process does at least two disk writes: it creates a PID file for the running process and also writes out the storage engine metadata file (storage.bson). For unit test suites with lots of tests, these file writes occur every time a test is run. This can add an undesirable amount of overhead to unit test execution, and these disk writes don't seem necessary since the unit tests mostly use an ephemeral storage engine. We should try to avoid doing these writes in our unit test fixtures.