WARNING: Here be dragons.

WARNING: Might sometimes work. Possibly.

You need to turn off class caching in environments/test.rb:

config.cache_classes = false

Gemfile:

group :test do
  gem 'guard-forku',
    git: 'https://github.com/dstrelau/guard-forku.git'
end

Once to generate a Guardfile:

$ bundle
$ bundle exec guard init forku

Start:

$ RUBYOPT=-Itest bundle exec guard