1. Create your gem:

Here is the most modern approach to gem crafting:

bundle gem gemname
cd gemname

edit your_gem.gemspec and add description, summary, required gems and optional website. Add required gems for development such as rspec, rails or minitest to the Gemfile.

2. Add testing framework:

RSpec:

rspec --init
touch spec/your_gem_spec.rb

Minitest:

# Rakefile

Getting started is easy!

Add this code:

Gemfile

gem "minitest-rails", github: "blowmage/minitest-rails"
gem "minitest-reporters"

application.rb

config.generators do |g|
  g.test_framework :minitest, spec: true, fixture: true
  g.helper false
  g.view_specs false
end

$> rails g minitest:install

test_helper.rb

Rails.env = "test"