[WIP] Added minitest plugin

[mfazekas]

This is more a request for comment than a pull req to merge.

It adds a minitest plugin so we can do stuff like rails test --benchmark to run perf test in benchmark mode.

It adds a minitest plugin that tries to get back the pre rails-4 feature of rake test not running performance tests. In all my projects perf test requires different db than test db, they also took a long time to run. So it's not practical to run them with every rake test.

In rails-4 world i've used a workaround like this:

  namespace :test do
    Rails::TestTask.new(:run) do |t|
      dirs = %w(models controllers integration mailers workers)
      t.test_files = dirs.map { |dir| Dir.glob("test/#{dir}/**/*_test.rb") }.flatten
    end
  end
  Rake::Task['test:run'].actions.shift

In rails 5 world this would be implemented by a minitest plugin. But i'm not sure who want's to run perf test together vs who doesn't. Maybe it should be a different rubygem. Like if you adding 'gem "rails-perftest-separate"' will run not run perftest on rails test run.
Otherwise i assume the rails test --benchmark can be usefull in rails-perftest.

[senny]

I guess most people do want to run perf tests separately. Having everything integrate with MiniTest seems nice as well.

That's just guessing though. I did the extraction from core into this gem but haven't used the functionality much.

I'd be 👍 to merge this. Since @kaspth worked on the test runner as well, maybe he has some insights of his own.

[kaspth]

I'm fine with adding this plugin. Though it should be said rails test's support for other plugins could stand to improve: rails/rails#25055. I.e. you shouldn't have to muck with the extension order.