tags: add more tests

Signed-off-by: Pierre-Alexandre Meyer <pierre@mouraf.org>
custom_fields: add tests

Signed-off-by: Pierre-Alexandre Meyer <pierre@mouraf.org>
.gitignore: ignore .DS_Store

Signed-off-by: Pierre-Alexandre Meyer <pierre@mouraf.org>
helpers: remove deprecated killbill_helper

Signed-off-by: Pierre-Alexandre Meyer <pierre@mouraf.org>