In Gemfile add gem 'test-unit'. Let bundler update Gemfile.lock.

Then in terminal run
$bundle install --path vendor/bundle
This shows test-unit version.
In Gemfile edit line to
gem 'test-unit', '>=2.0.0.0'

Note
gem separator is a dash 'test-unit'
require separator is a slash 'test/unit'

To run tests
$ bundle exec rake