Restore support library for only test files that are digest and csv.

test/csv/helper.rb

@@ -3,7 +3,7 @@ require "test/unit"
 
 require "csv"
 
-require_relative "../../tool/lib/with_different_ofs.rb"
+require_relative "../lib/with_different_ofs"
 
 module Helper
   def with_chunk_size(chunk_size)

test/digest/test_digest_extend.rb

@@ -1,7 +1,7 @@
 # frozen_string_literal: false
 require 'test/unit'
 require 'digest'
-require_relative '../../tool/lib/with_different_ofs.rb'
+require_relative '../lib/with_different_ofs'
 
 class TestDigestExtend < Test::Unit::TestCase
   extend DifferentOFS