The change to serialization mirrors the test just below it that also
uses a conditional for the assertion instead of a skip. The conditional
is necessary because memcached entries are not strings.

The class_serial test should not run on Ruby 3.2+ because class_serial
was replaced with Object Shapes. The class_serial value in RubyVM.stat
was removed in ruby/ruby@13bd617ea6fdf72467c593639cf33312a06c330c