Add additional scope check to Todo#for_target spec

This works around a condition where the Issue and the MergeRequest
created by FactoryBoth both had the same ID, which caused `for_target`
to return two records when we only expected one.

spec/models/todo_spec.rb

@@ -236,7 +236,8 @@ describe Todo do
 
       create(:todo, target: create(:merge_request))
 
-      expect(described_class.for_target(todo.target)).to eq([todo])
+      expect(described_class.for_type(Issue.name).for_target(todo.target))
+        .to contain_exactly(todo)
     end
   end