Config collections other than `config.system.sessions` are allowed to become sharded

For testing purposes, currently the config server allows any collection under the config database to become sharded when the testCommands flag has been enabled.

The collection filtering metadata logic however expects config collections to never be sharded and because of this invariants.

We should prohibit any config collections other than the known system ones to ever become sharded, regardless of whether the testCommands flag is enabled.