tests: Add support for "Chef tests", which are direct CLI tests. Since Chef tests are written as YAML tests and also involve creating a mock file-system, this change includes a refactor of the bake tests to put some functionality in a common base class.

#!/bin/sh

CUR_DIR="$( cd "$( dirname "$0" )" && pwd )"
CHEF=${CUR_DIR}/../bin/chef
OUT_DIR=${CUR_DIR}/../piecrust/resources/messages
ROOT_DIR=${CUR_DIR}/messages

$CHEF --root=$ROOT_DIR bake -o $OUT_DIR
rm ${OUT_DIR}/index.html