Fix around spec for MT5.

Around spec needs around unit!

After this commit the following code works again:

    require 'minitest/autorun'
    # NOTE the isolated usage
    require 'minitest/around/spec'

    describe "Mutex" do
      around do |test|
        puts "AROUND"
      end

      it "synchronized" do
        puts "SYNC"
      end
    end

`around` will be called again.

See https://github.com/splattael/minitest-around/issues/5#issuecomment-24951665