chore: Simplify Rakefile

Author: toshimaru

Rakefile

@@ -1,10 +1,8 @@
+# frozen_string_literal: true
+
 require "bundler/gem_tasks"
-require "rake/testtask"
+require "minitest/test_task"
 
-Rake::TestTask.new(:test) do |t|
-  t.libs << "test"
-  t.libs << "lib"
-  t.test_files = FileList["test/**/*_test.rb"]
-end
+Minitest::TestTask.create
 
-task :default => :test
+task default: :test