Require rubygems for ruby/ruby

You can't take rubygems for granted in a default gem.
https://github.com/ruby/ruby/actions/runs/3506561943/jobs/5873689466

Author: k0kubun

test/irb/test_cmd.rb

@@ -1,4 +1,5 @@
 # frozen_string_literal: false
+require "rubygems"
 require "irb"
 require "irb/extend-command"