Add instructions to run Integration Tests

What does this MR do?

Adds instructions to the test document that describe how to setup and run an integration test.

The MR also adds a new --test option that allows engineers to run an individual test using invoke.

What are the relevant issue numbers?

Requested in comment !275 (comment 404119760)

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated for GitLab EE, if necessary
• Documentation created/updated for this project, if necessary
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Job definition updated, if necessary
  • Job definition example
  • Vendored CI Templates (also in CE)
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer