foo.rb

require "minitest/autorun"

class Foo
  @option = "default"

  def self.build(value)
    klass = Class.new self
    klass.instance_variable_set :@option, value
    klass
  end

  def self.option
    @option
  end
end

def Foo(value)
  Foo.build value
end

class FooTest < Minitest::Test
  def test_builds_a_subclass
    klass = Foo "foo"
    assert klass < Foo, "is not a subclass"
  end

  def test_sets_option
    klass = Foo "bar"
    assert_equal "bar", klass.option
  end
end