Merge branch 'pl-doc-testing-guide-speed' into 'master'

Prefer `instance_double` in testing best practices

See merge request gitlab-org/gitlab!32085

doc/development/testing_guide/best_practices.md

@@ -23,10 +23,10 @@ and effective _as well as_ fast.
 
 Here are some things to keep in mind regarding test performance:
 
-- `double` and `spy` are faster than `FactoryBot.build(...)`
+- `instance_double` and `spy` are faster than `FactoryBot.build(...)`
 - `FactoryBot.build(...)` and `.build_stubbed` are faster than `.create`.
 - Don't `create` an object when `build`, `build_stubbed`, `attributes_for`,
-  `spy`, or `double` will do. Database persistence is slow!
+  `spy`, or `instance_double` will do. Database persistence is slow!
 - Don't mark a feature as requiring JavaScript (through `:js` in RSpec) unless it's _actually_ required for the test
   to be valid. Headless browser testing is slow!