The FSM (concurrency) tests cannot be invoked to execute from mongo (shell) against a running mongod, through the ScopedThread call, since ScopeThreads will not have a global db variable to use. Defining a db variable before loading fsm_all.js has no effect on other ScopedThreads (created by the FSM framework) because global variables are not shared between ScopeThreads.
The FSM framework assumes that a mongod was started by (re)smoke.py if sharding or replication isn't used by relying on the implicit db connection (i.e. fsm_all.js is incompatible with --nodb). This improvement is to Create a new "cluster" type that allows a specific port to be specified and gives that as the host the FSM worker threads will use.