Improve GraphQL testing guide

* Add a note about avoiding false positives in N+1 specs
* Add a note dictating the request spec directory structure

doc/development/api_graphql_styleguide.md

@@ -1381,6 +1381,62 @@ it 'returns a successful response' do
 end
 ```
 
+### Testing tips and tricks
+
+- Avoid false positives:
+
+  Authenticating a user with the `current_user:` argument for `post_graphql`
+  generates more queries on the first request than on subsequent requests on that
+  same user. If you are testing for N+1 queries using
+  [QueryRecorder](query_recorder.md), use a **different** user for each request.
+
+  The below example shows how a test for avoiding N+1 queries should look:
+
+  ```ruby
+  RSpec.describe 'Query.project(fullPath).pipelines' do
+    include GraphqlHelpers
+
+    let(:project) { create(:project) }
+
+    let(:query) do
+      %(
+        {
+          project(fullPath: "#{project.full_path}") {
+            pipelines {
+              nodes {
+                id
+              }
+            }
+          }
+        }
+      )
+    end
+
+    it 'avoids N+1 queries' do
+      first_user = create(:user)
+      second_user = create(:user)
+      create(:ci_pipeline, project: project)
+
+      control_count = ActiveRecord::QueryRecorder.new do
+        post_graphql(query, current_user: first_user)
+      end
+
+      create(:ci_pipeline, project: project)
+
+      expect do
+        post_graphql(query, current_user: second_user)  # use a different user to avoid a false positive from authentication queries
+      end.not_to exceed_query_limit(control_count)
+    end
+  end
+  ```
+
+- Mimic the folder structure of `app/graphql/types`:
+
+  For example, tests for fields on `Types::Ci::PipelineType`
+  in `app/graphql/types/ci/pipeline_type.rb` should live in
+  `spec/requests/api/graphql/ci/pipeline_spec.rb` regardless of the query being
+  used to fetch the pipeline data.
+
 ## Notes about Query flow and GraphQL infrastructure
 
 GitLab's GraphQL infrastructure can be found in `lib/gitlab/graphql`.