When a test finishes configuration, it's piped through the new stack interface. The stack controls what tests are being served to the client, maintaining a list of queued and completed tests. As soon as a test finishes, the next one is selected.
Now each test has it's own event emitter. This'll be useful for extensions and syntactic sugar, such as when a runner is ready to run the test, or when a test finishes on a client, or when it finishes on all clients, fail events, progress events, etc...
Instead of the Response constructor creating the test ID, that's been delegated to the test itself.
