Add a mailroom CI job to validate config
What does this MR do and why?
This job only attempts to load the MailRoom config, but does not actually try to start the process. This validation step would prevent issues as #468467 (closed) and #432257 (closed).
Note that this test uses Bundler even though Omnibus doesn't use it. A future improvement would be to run MailRoom with just the gem itself.
MR acceptance checklist
Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.
Screenshots or screen recordings
See https://gitlab.com/gitlab-org/gitlab/-/jobs/7208300720#L78 as a sample test job.
Edited by Stan Hu