Test runner updates

Yeh I looked at that. The builds [run in a goroutine](https://github.com/flynn/flynn/blob/master/test/runner/runner.go#L156-L177), so the events channel doesn't need the same capacity as the amount of concurrent builds as that loop is pretty quick.

I think I added a capacity to the events channel just so HTTP requests aren't blocked (i.e. [here](https://github.com/flynn/flynn/blob/master/test/runner/runner.go#L334)), but I'm not sure it's needed. WDYT?