Use test-unit-ruby-core instead of deep-copy from ruby repository (#65)

I and @nobu prepared https://github.com/ruby/test-unit-ruby-core for
`test-unit` with `EnvUtil`.

We should remove related files from this repository.

Author: hsbt

.github/workflows/ci.yml

@@ -88,6 +88,8 @@ jobs:
 
       - run: gem install --verbose --backtrace pkg/*.gem
 
+      - run: gem install test-unit-ruby-core
+
       - name: Run tests on the installed gem
         run: ruby run-test.rb
         if: matrix.ruby != '2.4' # strscan is a default gem from 2.5

Gemfile

@@ -7,4 +7,5 @@ group :development do
   gem "rake-compiler"
   gem "ruby-maven", :platforms => :jruby
   gem "test-unit"
+  gem "test-unit-ruby-core"
 end

Rakefile

@@ -44,13 +44,6 @@ task :benchmark do
        "benchmark/scan.yaml")
 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
-
 release_task = Rake.application["release"]
 release_task.prerequisites.delete("build")
 release_task.prerequisites.delete("release:rubygem_push")