Use test-unit gem instead of ruby core test suite

.github/workflows/ci.yml

@@ -22,8 +22,6 @@ jobs:
         - 2.7
         - 2.6
         - 2.5
-        - 2.4
-        - 2.3
         - debug
         include:
         - { os: ubuntu-20.04 ,   ruby: head }

Gemfile

@@ -1,6 +1,5 @@
 source "https://rubygems.org"
 
-git_source(:github) {|repo_name| "https://github.com/#{repo_name}" }
-
-# Specify your gem's dependencies in digest.gemspec
-gemspec
+gem "rake"
+gem "rake-compiler"
+gem "test-unit"

Rakefile

@@ -2,7 +2,8 @@ require "bundler/gem_tasks"
 require "rake/testtask"
 
 Rake::TestTask.new(:test) do |t|
-  ENV["RUBYOPT"] = "-Itest -Itest/lib -Ilib"
+  t.libs << "test" << "test/lib" << "lib"
+  t.ruby_opts << "-rhelper"
   t.test_files = FileList["test/**/test_*.rb"]
 end
 
@@ -12,4 +13,11 @@ Rake::ExtensionTask.new("digest")
   Rake::ExtensionTask.new("digest/#{ext}")
 end
 
+task :sync_tool do
+  require 'fileutils'
+  FileUtils.cp "../ruby/tool/lib/test/unit/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

digest.gemspec

@@ -26,7 +26,7 @@ Gem::Specification.new do |spec|
     "lib/digest.rb"
   ]
 
-  spec.required_ruby_version = ">= 2.3.0"
+  spec.required_ruby_version = ">= 2.5.0"
 
   spec.bindir        = "exe"
   spec.executables   = spec.files.grep(%r{^exe/}) { |f| File.basename(f) }
@@ -40,8 +40,4 @@ Gem::Specification.new do |spec|
     ext/digest/sha2/extconf.rb
   ]
   spec.metadata["msys2_mingw_dependencies"] = "openssl"
-
-  spec.add_development_dependency "bundler"
-  spec.add_development_dependency "rake"
-  spec.add_development_dependency "rake-compiler"
 end

test/digest/test_ractor.rb

@@ -14,7 +14,7 @@ module TestDigestRactor
   Data2 = "abcdbcdecdefdefgefghfghighijhijkijkljklmklmnlmnomnopnopq"
 
   def setup
-    skip unless defined?(Ractor)
+    pend unless defined?(Ractor)
   end
 
   def test_s_hexdigest