[ci skip] Improve upon testing tasks guide

Add missing testing tasks and also improve upon tasks descriptions
Merge pull request #17613 from sandipransing/testing-guide

[ci skip] Improve upon testing tasks guide
added ActiveRecord::Relation#outer_joins

Example:
  User.outer_joins(:posts)
  => SELECT "users".* FROM "users" LEFT OUTER JOIN "posts" ON "posts"."user_id" = "users"."id"