minitest-matchers + valid_attribute

post_spec.rb

# you have to use https://github.com/wojtekmach/valid_attribute/tree/minitest
require "minitest/autorun"
require "minitest/matchers"
require "active_model"
require "valid_attribute"

class Post
  include ActiveModel::Validations
  attr_accessor :title

  validates :title, :presence => true
end

describe Post do
  subject { Post.new }

  must { have_valid(:title).when("Good") }
  wont { have_valid(:title).when(nil, "") }
end