Merge pull request rails#49159 from jonathanhefner/fix-49143

Load ambient fixtures for `ActiveRecordMessagePackTest`

Author: jonathanhefner

activerecord/test/cases/message_pack_test.rb

@@ -9,6 +9,8 @@
 require "active_record/message_pack"
 
 class ActiveRecordMessagePackTest < ActiveRecord::TestCase
+  fixtures :posts, :comments, :authors
+
   test "enshrines type IDs" do
     expected = {
       119 => ActiveModel::Type::Binary::Data,