Unit tests on the config server must run the config server as a replica set via the ReplicationCoordinatorMock because our config server code expects (and frequently checks) that it is a replica set. The ephemeral storage engine invariants that the server is not a replica set in EphemeralForTestRecoveryUnit::setReadFromMajorityCommittedSnapshot. This must be changed to accept or reject majority writes depending on a flag passed down to it from when the ephemeral storage engine is initialized for the unit test.