Fix initial events handling for mux worker

* Rename `before_wait' worker callback to `before_watch';
* Setup `before_watch' callback in the runner, handling events with the
  manager;
* Test the mux worker with a cucumber scenario.

spec/uh/wm/runner_spec.rb

@@ -150,6 +150,11 @@ module Uh
           expect(runner.worker).to respond_to :work_events
         end
 
+        it 'setups the before_watch callback' do
+          expect(runner.manager).to receive :handle_pending_events
+          runner.worker.before_watch.call
+        end
+
         it 'setups the read callback' do
           expect(runner.manager).to receive :handle_pending_events
           runner.worker.on_read.call