Sorry! I haven't made it into a gem yet. Next thing on my list I promise.
require 'minitest/parallel' # optionally set the number of processors MiniTest::Parallel.processor_count = 2 # if you don't set the number of processors, it # defaults to the number of processors in your # system
You MUST require minitest/parallel before you require any other minitest files. MiniTest::Parallel is a monkeypatch, and therefore requires a specific version of the MiniTest gem to be activated.
MiniTest::Parallel runs your tests in parallel. That means if you try to access a shared object (like a database, or a third-party service) you need to make sure that you can interact with that services in a parallel manner.
I suggest making a separate directory of serial tests. Then, create two rake tasks, one for serial, and one for parallel. In the parallel task, require MiniTest::Parallel.