Extend long polling timeout for tests to avoid timeouts during travis runs due to slow running Firefox.

From examining detailed logs of failures, I've found instances where long polling obviously determines Firefox to be disconnected, and as a result calls can be hung up or dropped automatically when not intending it. Hence these can easily break tests and cause random failures.

In part, this should be fixed when we implement the [broken connections card](https://trello.com/c/dIPWHgvj), but for now at least we should extend the timeout and avoid issues.

This does not affect other tests - mocha tests already use fake timers and are keyed to the long polling timeout, and frontend tests don't test this part of the code.