Resolve "Add test infrastructure for Charts on dev"

Closes #1412 (closed)

Adds jobs for testing the chart using images from the dev registry.

Testing

• Jobs only trigger on *-stable branches in dev
  • Job in dev: https://dev.gitlab.org/gitlab/charts/gitlab/-/jobs/8394921
  • Pipeline in com with correctly skipped job: https://gitlab.com/gitlab-org/charts/gitlab/-/pipelines/174301731
• Helm chart deploys using images from the dev registry instead of .com
• imagePullSecret created with mirror bot successfully pulls images from dev
• Tests pass