Update testing.md `# Frontend Testing` to include integration spec for edge cases

What does this MR do?

EE port: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/2055

Adds to reasons to write an integration spec

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?