Reduce browsertests IRC spam

We have the browser tests results to always send a notification on IRC.
With monitoring going to spam #wikimedia-qa soon, reduce the amount of
trafic by only notifying state changes.

Note that we still notify everything to the qa-alerts mailling list.

Bug: 70778
Change-Id: I598e6a2be60adeb2d3bda8bb3fa7a50fd880eaeb