In verify_sessions_expiration_sharded.js, we refresh the logical session cache on mongos before guaranteeing that every config server node has replicated the chunk metadata associated with the sessions collection. We should either wait for the config server nodes to fully replicate the data or keep looping the mongos refresh until it is able to detect the sessions collection on the config server. 

Edit: Another(better) solution would be to move the cache refresh when starting up a ShardingTest above this line to guarantee that each of the config server nodes has refreshed their cache.