Don't modify existing test model in a test case

It will affect to other tests.

activerecord/test/cases/secure_token_test.rb

@@ -67,14 +67,13 @@ def test_token_length_cannot_be_less_than_24_characters
   end
 
   def test_token_on_callback
-    User.class_eval do
-      undef regenerate_token
-
+    model = Class.new(ActiveRecord::Base) do
+      self.table_name = "users"
       has_secure_token on: :initialize
     end
 
-    model = User.new
+    user = model.new
 
-    assert_predicate model.token, :present?
+    assert_predicate user.token, :present?
   end
 end