Speed up finders/merge_requests_finder_spec

Uses the :simple merge request factory trait introduced by d166e70; cuts
execution time of this spec in half.

spec/finders/merge_requests_finder_spec.rb

 require 'spec_helper'
 
 describe MergeRequestsFinder do
-  let(:user) { create :user }
+  let(:user)  { create :user }
   let(:user2) { create :user }
+
   let(:project1) { create(:project) }
   let(:project2) { create(:project) }
-  let(:merge_request1) { create(:merge_request, author: user, source_project: project1, target_project: project2) }
-  let(:merge_request2) { create(:merge_request, author: user, source_project: project2, target_project: project1) }
-  let(:merge_request3) { create(:merge_request, author: user, source_project: project2, target_project: project2) }
+
+  let!(:merge_request1) { create(:merge_request, :simple, author: user, source_project: project1, target_project: project2) }
+  let!(:merge_request2) { create(:merge_request, :simple, author: user, source_project: project2, target_project: project1) }
+  let!(:merge_request3) { create(:merge_request, :simple, author: user, source_project: project2, target_project: project2) }
 
   before do
     project1.team << [user, :master]
@@ -15,13 +17,7 @@ describe MergeRequestsFinder do
     project2.team << [user2, :developer]
   end
 
-  describe :execute do
-    before :each do
-      merge_request1
-      merge_request2
-      merge_request3
-    end
-
+  describe "#execute" do
     it 'should filter by scope' do
       params = { scope: 'authored', state: 'opened' }
       merge_requests = MergeRequestsFinder.new.execute(user, params)