Resolve "Add tests and validation logic to greenhouse integration"

Why is this change being made?

The Greenhouse integration code is brittle. It has broken all builds on www-gitlab-com- see slack thread in #6071 (closed).

Per @stanhu 's suggestion, it needs to be improved and tests need to be added. In particular: https://gitlab.com/gitlab-com/www-gitlab-com/blob/master/helpers/jobs_helpers.rb

Does this MR meet the acceptance criteria?

Conformity

• Added description to this MR explaining the reasons for the proposed change, per say-why-not-just-what

Closes #6071 (closed)