Gemfile

source "https://rubygems.org"

gem "whatever"

# Load minitest and red-green
group :minitest do
  gem "minitest"
  gem "minitest-rg"
end

run_tests.rb

#!/usr/bin/env ruby
# Put this in bin/run_tests.rb or something and chmod 755 it

# Load bundler
require "bundler/setup"
Bundler.require :default, :minitest

# Load your code
require_relative "../lib/my_gem.rb"

# Run minitest with ARGV
Dir["tests/**/*.rb"].each &method(:load)
Minitest.run ARGV