[ruby/irb] Deal with inconsistency with ruby/ruby

https://github.com/ruby/irb/commit/41d5012849

test/irb/helper.rb

@@ -1,4 +1,8 @@
 require "test/unit"
+begin
+  require_relative "../lib/helper"
+rescue LoadError # ruby/ruby defines helpers differently
+end
 
 module IRB
   class InputMethod; end