This becomes more relevant as sharding catalog operations move to being performed locally by the config server rather than over the network.

While sharding catalog operations were performed by mongos against the config server, unit tests could use the mock network interface's "onCommand*" methods to "expect" (block and wait for) certain reads/writes to be sent over the network from mongos to the config server.

Now that mongod performs these operations locally, it'd be nice to have a way to tell the mock storage engine to similarly "expect" read/write operations to be performed against it.

It may also be a valuable feature for unit testing transactions and retriable writes local read/write logic.