Add importers scenario type

What does this MR do and why?

This adds a new scenario type for testing various gitlab importers.

• second gitlab instance for gitlab migration functionality testing
• mock server to mock out any other external source for import

Part of: gitlab#382166 (closed)

How to set up and validate locally

bundle exec gitlab-qa Test::Integration::Import EE

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.