Use to_reference for issue and merge request mentionable specs

spec/models/issue_spec.rb

@@ -70,7 +70,7 @@ describe Issue do
   it_behaves_like 'an editable mentionable' do
     subject { create(:issue, project: project) }
-    let(:backref_text) { "issue ##{subject.iid}" }
+    let(:backref_text) { "issue #{subject.to_reference}" }
     let(:set_mentionable_text) { ->(txt){ subject.description = txt } }
   end

spec/models/merge_request_spec.rb

@@ -165,10 +165,10 @@ describe MergeRequest do
   end
   it_behaves_like 'an editable mentionable' do
-    subject { create(:merge_request, source_project: project, target_project: project) }
-    let(:backref_text) { "merge request !#{subject.iid}" }
-    let(:set_mentionable_text) { ->(txt){ subject.title = txt } }
+    subject { create(:merge_request, source_project: project) }
+    let(:backref_text) { "merge request #{subject.to_reference}" }
+    let(:set_mentionable_text) { ->(txt){ subject.description = txt } }
   end
   it_behaves_like 'a Taskable' do