Update testing guide to reference test design

What does this MR do?

• Add references to the Testing Guide doc to test design
• Add summary and link in Testing Best Practices doc to test design heuristics documentation

What are the relevant issue numbers?

gitlab-org/quality/team-tasks#88

Does this MR meet the acceptance criteria?

• [-] Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• [-] Tests added for this feature/bug
• [-] Tested in all supported browsers
• Conforms to the code review guidelines
• [-] Conforms to the merge request performance guidelines
• Conforms to the style guides
• [-] Conforms to the database guides
• [-] Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer