Load ambient fixtures for ActiveRecordMessagePackTest

`ActiveRecordMessagePackTest` does not use any fixtures, but fixtures
that were previously loaded without preserving referential integrity can
affect query results.  For example, if previous tests load `:comments`
fixtures but not `:posts` fixtures, there will be `Comment` records in
the database with `post_id = 1` but no corresponding `Post` record.
Then, when a `Post` is created in `ActiveRecordMessagePackTest`, its
`comments` association will accidentally include those prior `Comment`
records.

This commit loads ambient fixtures for the relevant models in
`ActiveRecordMessagePackTest` to prevent such accidental results.

Fixes rails#49143.

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,