Fix dialogue UI js test timeout

[This particular `.set`](https://github.com/praekelt/vumi-go/blob/develop/go/base/static/js/test/apps/dialogue/states/send.test.js#L24-L32) causes its enclosing `beforeEach` to take quite long, often timing out tests. This is because clearing the old states and adding a new state causes the relevant views listening to the model to update, resulting in quite a few DOM updates, which takes a while. Since these view updates are unnecessary for the purpose of the `beforeEach`, we can make the `.set` silent.

In chrome, this was taking ~150ms. In phantomjs, this was taking ~500ms, so this is more an issue in tests than it is in the UI, though it still a significant amount of time. Such a drastic change isn't really likely with the way the UI is used I don't think, that particular `.set` causes an 'all at once' remove of all current states and create of a new state, each remove requiring unhooking and removing of views for connections and states, and the relevant DOM changes for each. After making the `.set` silent, this took ~50ms in chrome and ~100ms in phantomjs (still longer than it should be, but much quicker, enough to avoid test timeouts). I suspect the remaining delay is because of backbone-relational managing all the removed connection and state models, also something unlikely to be done in the UI all at once, I think.