fix transient test failures

test/serializers/cache_test.rb

@@ -72,16 +72,19 @@ module ActiveModel
         assert_equal(nil, ActionController::Base.cache_store.fetch(@post.cache_key))
         assert_equal(nil, ActionController::Base.cache_store.fetch(@comment.cache_key))
 
+        Timecop.freeze(Time.now) do
           post = render_object_with_cache(@post)
 
           assert_equal(@post_serializer.attributes, ActionController::Base.cache_store.fetch(@post.cache_key))
           assert_equal(@comment_serializer.attributes, ActionController::Base.cache_store.fetch(@comment.cache_key))
         end
+      end
 
       def test_associations_cache_when_updated
         # Clean the Cache
         ActionController::Base.cache_store.clear
 
+        Timecop.freeze(Time.now) do
           # Generate a new Cache of Post object and each objects related to it.
           render_object_with_cache(@post)
 
@@ -101,6 +104,7 @@ module ActiveModel
           assert_equal(new_comment_serializer.attributes, ActionController::Base.cache_store.fetch(new_comment.cache_key))
           assert_equal(@post_serializer.attributes, ActionController::Base.cache_store.fetch(@post.cache_key))
         end
+      end
 
       def test_fragment_fetch_with_virtual_associations
         expected_result = {