You need to tell RuboCop to load the Minitest extension. There are three ways to do this:

RuboCop configuration file

Put this into your .rubocop.yml.

require: rubocop-minitest

Now you can run rubocop and it will automatically load the RuboCop Minitest cops together with the standard cops.

Command line

rubocop --require rubocop-minitest

Rake task do |task|
  task.requires << 'rubocop-minitest'