Minitest running twice when dealing with forks

minitest_twice_fork.rb

require 'minitest/autorun'

class TestSuite
  def self.before_run
    pid = fork do
      puts "on fork"
      exit 0
    end

    Process.detach pid
  end
end

class MyTestCase < Minitest::Test
  TestSuite.before_run
end

class TestTests < MyTestCase
  def test_aa
    puts "aaa"
  end
end

The reason for this duplication is that Minitest uses #at_exit to write results (is the only way to deal with exits inside application); To workaround this, calls to Kernel#exit should be replaced by Kernel#exit!, which ignore callbacks).