add 6-1 marshal fixture

Files where generated by running the following spec in 6-1-stable
branch:

```
class MarshalSerializationTest < ActiveRecord::TestCase
  fixtures :topics

  def test_deserializing_rails_6_0_marshal_basic
    File.binwrite(marshal_fixture_path("rails_6_1_topic"),
Marshal.dump(Topic.find(1)))
    topic = Topic.find(1)

    topic.replies
    File.binwrite(marshal_fixture_path("rails_6_1_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
```

Author: mgrunberg