Bug 1042197 - [Email] Close IMAP connections...

It seems like it would be more rigorous for us to do something like in test_incoming_prober.js where we manually control the timeout firing rather than adjusting the timeout to zero.  In this case I think it could make a lot of sense because it seems like there is an inherent race in changing the timeout here between the other steps.  It seems like you're expecting this change to end up killing the previous connection's lingering connection sometime during the next step.

If you controlled the timeout, you could just have a T.action() step where you fire the timer and then observe that the connection dies.