Use test-unit-ruby-core instead of hard copy from ruby repository

Author: hsbt

Gemfile

@@ -9,4 +9,5 @@ group :development do
   gem "bundler"
   gem "rake"
   gem "test-unit"
+  gem "test-unit-ruby-core"
 end

Rakefile

@@ -15,11 +15,4 @@ Rake::TestTask.new(:test) do |t|
   t.test_files = FileList["test/**/test_*.rb"]
 end
 
-task :sync_tool do
-  require 'fileutils'
-  FileUtils.cp "../ruby/tool/lib/core_assertions.rb", "./test/lib"
-  FileUtils.cp "../ruby/tool/lib/envutil.rb", "./test/lib"
-  FileUtils.cp "../ruby/tool/lib/find_executable.rb", "./test/lib"
-end
-
 task :default => [:test]