[ruby/irb] Rename test_helper.rb to helper.rb

The name test_helper.rb conflicts with the test_helper.rb in JSON,
causing build failures. This commit renames test_helper.rb to helper.rb.

https://github.com/ruby/irb/commit/b6a92bf6b3

Co-Authored-By: Stan Lo <stan001212@gmail.com>

test/irb/test_cmd.rb

@@ -3,7 +3,7 @@ require "test/unit"
 require "irb"
 require "irb/extend-command"
 
-require_relative "test_helper"
+require_relative "helper"
 
 module TestIRB
   class ExtendCommand < Test::Unit::TestCase

test/irb/test_input_method.rb

@@ -3,7 +3,7 @@
 require "test/unit"
 require "irb"
 
-require_relative "test_helper"
+require_relative "helper"
 
 module TestIRB
   class TestRelineInputMethod < Test::Unit::TestCase