Sidekiq configuration test

[ferblape]

Rel: https://github.com/PopulateTools/gobierto-contratos/issues/1497

Create a test that checks:

• there's a queue named mailer in the config/sidekiq.yml file
• all the queues used in classes from app/jobs are present in config/sidekiq.yml file

This test will prevent us to producers enqueuing jobs in queues without consumers.