Add some tests to Puma::Events (#1161)

* Add some tests to Puma::Events class.

* Add missing require to Puma::Events.

lib/puma/events.rb

@@ -1,4 +1,5 @@
 require 'puma/const'
+require "puma/null_io"
 require 'stringio'
 
 module Puma

test/test_events.rb

@@ -0,0 +1,58 @@
+require "test_helper"
+
+require "puma/events"
+
+class TestEvents < Minitest::Test
+  def test_null
+    events = Puma::Events.null
+
+    assert_kind_of Puma::NullIO, events.stdout
+    assert_kind_of Puma::NullIO, events.stderr
+  end
+
+  def test_strings
+    events = Puma::Events.strings
+
+    assert_kind_of StringIO, events.stdout
+    assert_kind_of StringIO, events.stderr
+  end
+
+  def test_stdio
+    events = Puma::Events.stdio
+
+    assert_equal STDOUT, events.stdout
+    assert_equal STDERR, events.stderr
+  end
+
+  def test_register_callback_with_block
+    res = false
+
+    events = Puma::Events.null
+
+    events.register(:exec) { res = true }
+
+    events.fire(:exec)
+
+    assert_equal true, res
+  end
+
+  def test_register_callback_with_object
+    obj = Object.new
+
+    def obj.res
+      @res || false
+    end
+
+    def obj.call
+      @res = true
+    end
+
+    events = Puma::Events.null
+
+    events.register(:exec, obj)
+
+    events.fire(:exec)
+
+    assert_equal true, obj.res
+  end
+end