assertion_extensions.rb

module AssertionExtensions
  def method_missing(method_id, *arguments, &block)
    return super unless method_id.to_s =~ /^assert_(not_)?(.*)$/

    method = "#{$2}?"
    object = arguments.first

    if $1
      arguments.each do |object|
        assert ! object.send(method), "#{method} is not false for #{object}"
      end
    else
      arguments.each do |object|
        assert object.send(method), "#{method} is not true for #{object}"
      end
    end
  end
end

class ActiveSupport::TestCase
  include AssertionExtensions
end

person_test.rb

class PersonTest < ActiveSupport::TestCase
  def test_is_admin
    assert_admin people(:admin), people(:superuser)
    assert_not_admin people(:foo), people(:guest)
  end
end