Update fe_guide testing.md

What does this MR do?

ee port: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1915

Update fe_guide testing.md to include some distinction between jasmine unit and integration, added stubbing practises, test block functions practises and an example 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?