Include JH spec jobs in code coverage job

Related upstream issue , Support merging dependencies arrays with !reference

Background

A short term solution has been implemented in #1642 (closed), and we are going to use extend instead of copy-paste.

Proposal

• add a .coverage-extra-dependencies in .gitlab/ci/rails.gitlab-ci.yml
• override .coverage-extra-dependencies in jh/.gitlab/ci/rails.gitlab-ci.yml