Wait for node to start stepdown before killing shutdown operation in force_shutdown_primary.js

In force_shutdown_primary.js, we may kill the shutdown operation before we actually start attempting to step the primary down. As a result, the parallel shell can exit cleanly with an exit code of zero, when we expect it to have a nonzero code. We should add a failpoint right before stepdown, and wait for the primary to reach that failpoint before we attempt to kill the shutdown operation.