[ruby/irb] Require rubygems to run the test alone on ruby/ruby

`Gem` is not undefined on test-all

https://github.com/ruby/irb/commit/08ac803d61

test/irb/test_ruby_lex.rb

@@ -1,5 +1,6 @@
 $LOAD_PATH.unshift File.expand_path('../../lib', __FILE__)
 require 'irb'
+require 'rubygems'
 require 'test/unit'
 require 'ostruct'