minitest + valid_attribute

post_test.rb

gem "minitest"
require "minitest/spec"
require "minitest/autorun"
require "active_model"
require "test/unit"
require "valid_attribute"
require "turn"

class MiniTest::Unit::TestCase
  extend ValidAttribute::Method
end

class Post
  attr_accessor :title

  include ActiveModel::Validations
  validates :title, :presence => true, :length => 2..16
end

class MiniTest::Spec
  class << self
    def it_must &block
      matcher = yield

      it "must #{matcher.description}" do
        result = matcher.matches? subject
        assert result, matcher.failure_message
      end
    end

    def it_wont &block
      matcher = yield

      it "wont #{matcher.description}" do
        result = matcher.does_not_match? subject
        assert result, matcher.negative_failure_message
      end
    end
  end
end

describe "Post" do
  subject { Post.new }

  it_must { have_valid(:title).when("Hello") }
  it_wont { have_valid(:title).when(nil, "", "X") }
end

I created a separate issue on the minitest project page minitest/minitest#33 so we'll see how it goes there.
The way i see it we have two options to continue:
a) Minitest itself will have a support for matchers (included or via a gem)
b) Something along these lines:

module ValidAttribute::MiniTest
  def must_have_valid attr ; end
  def wont_have_valid attr ; end
end

describe "Post" do
  extend ValidAttribute::MiniTest
  subject { Post.new }

  must_have_valid(:title).when("Hello")
end

That extend ValidAttribute::MiniTest can of course be pushed inside MiniTest::Spec