Copy also helper.rb [ci skip]

tool/lib/helper.rb

@@ -0,0 +1,4 @@
+require "test/unit"
+require_relative "core_assertions"
+
+Test::Unit::TestCase.include Test::Unit::CoreAssertions

tool/sync_test_lib.rb

@@ -6,6 +6,7 @@ test_lib_files = %w[
   core_assertions.rb
   find_executable.rb
   envutil.rb
+  helper.rb
 ].map do |file|
   [file, File.read("#{__dir__}/lib/#{file}")]
 end