Replace anonymous class in ActsAsOpEngine spec

Rails 4.2 is not able generate an appropriate `railtie_name` (aliased
to `engine_name`) for anonymous classes - this, in turn, breaks
initializer definitions.

Signed-off-by: Alex Coles <alex@alexbcoles.com>

spec/lib/open_project/plugins/acts_as_op_engine_spec.rb

@@ -30,12 +30,12 @@ require 'spec_helper'
 require 'roar/decorator'
 
 describe OpenProject::Plugins::ActsAsOpEngine do
-  subject(:engine) do
-    Class.new(Rails::Engine) do
-      include OpenProject::Plugins::ActsAsOpEngine
-    end
+  class ActsAsOpEngineTestEngine < Rails::Engine
+    include OpenProject::Plugins::ActsAsOpEngine
   end
 
+  subject(:engine) { ActsAsOpEngineTestEngine }
+
   it { is_expected.to respond_to(:name) }
   it { is_expected.to respond_to(:patches) }
   it { is_expected.to respond_to(:assets) }
@@ -43,13 +43,9 @@ describe OpenProject::Plugins::ActsAsOpEngine do
   it { is_expected.to respond_to(:register) }
 
   describe '#name' do
-    before do
-      Object.const_set('SuperCaliFragilisticExpialidocious', engine)
-    end
-
     describe '#name' do
-      subject { super().name }
-      it { is_expected.to eq 'SuperCaliFragilisticExpialidocious' }
+      subject { engine.name }
+      it { is_expected.to eq 'ActsAsOpEngineTestEngine' }
     end
   end