Minitest in spec directory

minitest.rake

require 'rake/testtask'                                                                                                                                                       
                                                                                                                                                                             
Rake::TestTask.new do |test_task|                                                                                                                                             
  test_task.libs.push 'spec'                                                                                                                                                  
  test_task.name = 'spec'                                                                                                                                                     
  test_task.pattern = 'spec/**/*_spec.rb'                                                                                                                                     
  test_task.verbose = true                                                                                                                                                    
end                                                                                                                                                                           
                                                                                                                                                                                 
task default: :spec                                                                                                                                                           
                    

If using Rails with a database you'll want, Rake::TestTask.new(test: 'db:test:prepare')