Fix Events#fire_on_stopped! double call, add test (puma#2497)

Launcher#graceful_stop calls Events#fire_on_stopped!, and graceful_stop is called in Launcher#run

Added TestLauncher#test_fire_on_stopped

lib/puma/launcher.rb

@@ -139,7 +139,6 @@ def halt
 
     # Begin async shutdown of the server gracefully
     def stop
-      @events.fire_on_stopped!
       @status = :stop
       @runner.stop
     end

test/test_launcher.rb

@@ -174,6 +174,26 @@ def test_log_config_disabled
     refute_match(/Configuration:/, launcher.events.stdout.string)
   end
 
+  def test_fire_on_stopped
+    conf = Puma::Configuration.new do |c|
+      c.app -> {[200, {}, ['']]}
+      c.port UniquePort.call
+    end
+
+    launcher = launcher(conf)
+    launcher.events.on_booted {
+      sleep 1.1 unless Puma.mri?
+      launcher.stop
+    }
+    launcher.events.on_stopped { puts 'on_stopped called' }
+
+    out, = capture_io do
+      launcher.run
+    end
+    sleep 0.2 unless Puma.mri?
+    assert_equal 'on_stopped called', out.strip
+  end
+
   private
 
   def events