backup_restore*.js tests send SIGTERM to resmoke.py and may leak mongo shell running FSM workloads

Sending a SIGTERM to resmoke.py causes the Python process to immediately exit (it doesn't register a handler for that signal like the server does) without waiting for any of the mongo shell processes it spawned to also exit (though they also receive the SIGTERM signal). This means if resmoke.py was in the midst of spawning a mongo shell process to run an FSM workload, then it could continue to run (i.e. be leaked) even after the parent resmoke.py process exits.