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.

features/workers/mux.feature

@@ -0,0 +1,15 @@
+Feature: multiplexing worker
+
+  Scenario: processes initial events
+    Given uhwm is running with options -d -w mux
+    Then the output must match /xevent/i at least 2 times
+
+  Scenario: processes generated events
+    Given a run control file with:
+      """
+      key(:f) { puts 'testing_worker_read' }
+      """
+    And uhwm is running with options -d -w mux
+    When I press the alt+f key 3 times
+    And I quit uhwm
+    Then the output must match /(testing_worker_read)/ exactly 3 times

lib/uh/wm/runner.rb

@@ -58,6 +58,9 @@ module Uh
 
       def worker
         @worker ||= Workers.build(*(@env.worker)).tap do |w|
+          w.before_watch do
+            @manager.handle_pending_events
+          end
           w.on_read do
             @manager.handle_pending_events
           end

lib/uh/wm/workers/base.rb

@@ -2,7 +2,7 @@ module Uh
   module WM
     module Workers
       class Base
-        CALLBACKS = %w[before_wait on_timeout on_read on_read_next].freeze
+        CALLBACKS = %w[before_watch on_timeout on_read on_read_next].freeze
 
         def initialize **options
           @ios = []

lib/uh/wm/workers/mux.rb

@@ -8,7 +8,7 @@ module Uh
         end
 
         def work_events
-          @before_wait.call if @before_wait
+          @before_watch.call if @before_watch
           if res = select(@ios, [], [], @timeout) then @on_read.call res
           else @on_timeout.call if @on_timeout end
         end

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