Fix local test failure

[JulienVig]

Running server tests fails with a timeout on 1) end-to-end federated "two cifar10 users reach consensus", the user accuracies don't converge. (At the time of writing, on the develop branch commit c0f27cf)

[JulienVig]

The mocha test timeout was simply too low for my humble laptop, increasing it from 1.5 min to 2 min solves the local test failure.