Clarify test
Refactor Test.runnable methods

- Move method order depending on test_order to a private method
Change spec method prefix from "test_" to "spec_"

Every subclass of Runnable should be able to know how to run it's own
tests.
Move .runnable_methods and PREFIX to Spec class
Rename DSL#create to DSL#create_sub_klass
Refactor: extract regexp to variable