Assertion 'created_directory' eagerly removes directory before other assertions are checked

[andreasabel]

Assertion 'created_directory' eagerly removes directory before other assertions are checked. E.g. you cannot place the created_directory assertion before the created_file assertion here: https://github.com/fugue/goldplate/blob/a2c20310149a69522d0efc2cdcd8e1081f5f93b4/tests/create.goldplate#L9-L13

A bit too imperative for my taste. I would have expected a declarative approach to these assertions, so that the order in which I write these do not matter.