Add a "how to improve this test?" section in issue description
Context
As part of gitlab#413963 (closed), I'm working on gitlab#414929 (closed) to test-drive the docs to improve test speed (https://docs.gitlab.com/ee/development/testing_guide/best_practices.html#test-speed).
I noticed that the message in the issue doesn't tell us on how to improve the specs performance.
What's in this MR?
Add a section in the issue on how to actually improve the test performance.