brew-test-bot: allow default_formula fallback in custom requirements

With this brew-test-bot doesn't skip builds, if a requirement isn't satisfied but a default_formula is specified. This would allow brew-test-bot to test #25276 with the custom python273requirement and automatiucly fall back to a homebrewed python on os x 10.8-.

This is done by checking if all requirements of the formula are satisfied. If not and a default formula is specified in the requirement, it recursively checks if the requirements of this default formula are satisfied or another default formula is specified…

This default_formula is already in dependencies, because of brew deps, so we doesn’t need to handle this here.