Rails 6.1: Add Active Record Marshal forward compatibility tests

[mgrunberg]

Rails add Marshal compatibility tests (rails/rails@617f990).

This PR adds the missing dump files. It also coerce the tests to load files from a path relative to ARTest::SQLServer.test_root_sqlserver.

Files where generated by running the following spec in 6-0-stable branch:

class MarshalSerializationTest < ActiveRecord::TestCase
  fixtures :topics

  def test_deserializing_rails_6_0_marshal_basic
    File.binwrite(marshal_fixture_path("rails_6_0_topic"), Marshal.dump(Topic.find(1)))
    topic = Topic.find(1)

    topic.replies
    File.binwrite(marshal_fixture_path("rails_6_0_topic_associations"), Marshal.dump(topic))
  end

  private
    def marshal_fixture_path(file_name)
      File.expand_path(
        "support/marshal_compatibility_fixtures/#{ActiveRecord::Base.connection.adapter_name}/#{file_name}.dump",
        ARTest::SQLServer.test_root_sqlserver
      )
    end
end