Refactor Rakefile for improved test task configuration

- Update the test task in Rakefile to include all files ending with "_test.rb" in the "test" directory
- Change the default task to only run the test task

Rakefile

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