The test currently runs replSetStepDown with a short stepdownTime of 1 second to 
make it more likely for the old primary to step back up. However, if the old primary spends more than 1 second to kill background operations while stepping down, the replSetStepdown will fail with LockTimeout since when force: true, the stepdownTime is the lock timeout for the RSTL (ReplicationStateTransition lock). To avoid this error, we should instead run replSetStepDown with stepdownTime of ReplSetTest.kForeverSecs and immediately then run a

command so the old primary is immediately eligible to step back up.