Ensure multiple anonymous modules are not included into Topic in tests

Each acceptance validator applied to a model class includes an instance
of a module builder (LazilyDefineAttributes) into that class. In tests,
if the original model class is not subclassed, these modules pile up and
cannot be removed, potentially leading to flakey specs and false
positive/negatives.

To avoid this, always use subclasses in tests whose names (constants)
can be removed when the test is done.

activemodel/test/cases/validations/acceptance_validation_test.rb

@@ -7,21 +7,23 @@
 require "models/person"
 
 class AcceptanceValidationTest < ActiveModel::TestCase
-  def teardown
-    Topic.clear_validators!
+  teardown do
+    self.class.send(:remove_const, :TestClass)
   end
 
   def test_terms_of_service_agreement_no_acceptance
-    Topic.validates_acceptance_of(:terms_of_service)
+    klass = define_test_class(Topic)
+    klass.validates_acceptance_of(:terms_of_service)
 
-    t = Topic.new("title" => "We should not be confirmed")
+    t = klass.new("title" => "We should not be confirmed")
     assert_predicate t, :valid?
   end
 
   def test_terms_of_service_agreement
-    Topic.validates_acceptance_of(:terms_of_service)
+    klass = define_test_class(Topic)
+    klass.validates_acceptance_of(:terms_of_service)
 
-    t = Topic.new("title" => "We should be confirmed", "terms_of_service" => "")
+    t = klass.new("title" => "We should be confirmed", "terms_of_service" => "")
     assert_predicate t, :invalid?
     assert_equal ["must be accepted"], t.errors[:terms_of_service]
 
@@ -30,9 +32,10 @@ def test_terms_of_service_agreement
   end
 
   def test_eula
-    Topic.validates_acceptance_of(:eula, message: "must be abided")
+    klass = define_test_class(Topic)
+    klass.validates_acceptance_of(:eula, message: "must be abided")
 
-    t = Topic.new("title" => "We should be confirmed", "eula" => "")
+    t = klass.new("title" => "We should be confirmed", "eula" => "")
     assert_predicate t, :invalid?
     assert_equal ["must be abided"], t.errors[:eula]
 
@@ -41,9 +44,10 @@ def test_eula
   end
 
   def test_terms_of_service_agreement_with_accept_value
-    Topic.validates_acceptance_of(:terms_of_service, accept: "I agree.")
+    klass = define_test_class(Topic)
+    klass.validates_acceptance_of(:terms_of_service, accept: "I agree.")
 
-    t = Topic.new("title" => "We should be confirmed", "terms_of_service" => "")
+    t = klass.new("title" => "We should be confirmed", "terms_of_service" => "")
     assert_predicate t, :invalid?
     assert_equal ["must be accepted"], t.errors[:terms_of_service]
 
@@ -52,9 +56,10 @@ def test_terms_of_service_agreement_with_accept_value
   end
 
   def test_terms_of_service_agreement_with_multiple_accept_values
-    Topic.validates_acceptance_of(:terms_of_service, accept: [1, "I concur."])
+    klass = define_test_class(Topic)
+    klass.validates_acceptance_of(:terms_of_service, accept: [1, "I concur."])
 
-    t = Topic.new("title" => "We should be confirmed", "terms_of_service" => "")
+    t = klass.new("title" => "We should be confirmed", "terms_of_service" => "")
     assert_predicate t, :invalid?
     assert_equal ["must be accepted"], t.errors[:terms_of_service]
 
@@ -66,23 +71,32 @@ def test_terms_of_service_agreement_with_multiple_accept_values
   end
 
   def test_validates_acceptance_of_for_ruby_class
-    Person.validates_acceptance_of :karma
+    klass = define_test_class(Person)
+    klass.validates_acceptance_of :karma
 
-    p = Person.new
+    p = klass.new
     p.karma = ""
 
     assert_predicate p, :invalid?
     assert_equal ["must be accepted"], p.errors[:karma]
 
     p.karma = "1"
     assert_predicate p, :valid?
-  ensure
-    Person.clear_validators!
   end
 
   def test_validates_acceptance_of_true
-    Topic.validates_acceptance_of(:terms_of_service)
+    klass = define_test_class(Topic)
+    klass.validates_acceptance_of(:terms_of_service)
 
-    assert_predicate Topic.new(terms_of_service: true), :valid?
+    assert_predicate klass.new(terms_of_service: true), :valid?
   end
+
+  private
+
+    # Acceptance validator includes anonymous module into class, which cannot
+    # be cleared, so to avoid multiple inclusions we use a named subclass which
+    # we can remove in teardown.
+    def define_test_class(parent)
+      self.class.const_set(:TestClass, Class.new(parent))
+    end
 end