The timeoutMS that the test uses to wait for a mongos' RSM to track that a node that stepped down is a secondary needs to be > 10 seconds.

The reason is that after a primary steps down and sends a hello to the mongos' RSM indicating it is a secondary, a concurrent background thread can fail to run a command due to stepdown. This then causes the RSM to overwrite the node's state from secondary to unknown. The test will then need to wait > 10 seconds so that the mongos' RSM can receive another hello from the stepped down primary indicating it is a secondary. 

See BF-29287 for more information.