Revert "Simplify Rakefile"

This reverts commit 5e45982.

Reason: Old minitest doesn't support `minitest/test_task` and I would like to support old Ruby versions.

Rakefile

@@ -1,10 +1,12 @@
-# frozen_string_literal: true
-
 require "bundler/gem_tasks"
-require "minitest/test_task"
+require "rake/testtask"
 
-Minitest::TestTask.create do |t|
+Rake::TestTask.new(:test) do |t|
+  t.libs << "test"
+  t.libs << "lib"
+  t.test_files = FileList['test/**/*_test.rb']
   t.verbose = true
+  t.warning = true
 end
 
-task default: :test
+task :default => :test