Add and Remove tests for FollowRequest (#5622)

* Add a test for FollowRequest#authorize!

* Remove tests

There is no need to test
ActiveModel::Validations::ClassMethods#validates.

* Make an alias of destroy! as reject!

Instead of defining the method,
make an alias of destroy! as reject! because of reducing test.

app/models/follow_request.rb

@@ -27,7 +27,5 @@ class FollowRequest < ApplicationRecord
     destroy!
   end
 
-  def reject!
-    destroy!
-  end
+  alias reject! destroy!
 end