Delete test cases when migrating project to a free group

CI test cases are only available for ultimate hosted
plans hence we should delete them when the project is transferred
to a lower plan.

Changelog: fixed
MR: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/67374
EE: true

doc/user/project/settings/index.md

@@ -362,6 +362,14 @@ NOTE:
 GitLab administrators can use the administration interface to move any project to any
 namespace if needed.
 
+##### Transferring a GitLab.com project to a different subscription tier
+
+When you transfer a project from a namespace that's licensed for GitLab SaaS Premium or Ultimate to Free, some data related to the paid features is deleted.
+
+For example, [project access tokens](../../../user/project/settings/project_access_tokens.md) are revoked, and
+[pipeline subscriptions](../../../ci/pipelines/multi_project_pipelines.md#trigger-a-pipeline-when-an-upstream-project-is-rebuilt)
+and [test cases](../../../ci/test_cases/index.md) are deleted.
+
 #### Delete a project
 
 You can mark a project to be deleted.

ee/app/services/ee/projects/transfer_service.rb

@@ -50,6 +50,7 @@ module EE
       def remove_paid_features
         revoke_project_access_tokens
         delete_pipeline_subscriptions
+        delete_test_cases
       end
 
       def revoke_project_access_tokens
@@ -66,6 +67,12 @@ module EE
 
         project.upstream_project_subscriptions.destroy_all # rubocop: disable Cop/DestroyAll
       end
+
+      def delete_test_cases
+        return if new_namespace.licensed_feature_available?(:quality_management)
+
+        project.issues.with_issue_type(:test_case).delete_all
+      end
     end
   end
 end

ee/spec/services/projects/transfer_service_spec.rb

@@ -204,5 +204,26 @@ RSpec.describe Projects::TransferService do
         end
       end
     end
+
+    describe 'test cases' do
+      before do
+        create(:quality_test_case, project: project, author: user)
+        stub_ee_application_setting(should_check_namespace_plan: true)
+      end
+
+      context 'when target namespace has a premium plan' do
+        it 'does not delete the test cases' do
+          subject.execute(premium_group)
+
+          expect { subject.execute(premium_group) }.not_to change { project.issues.with_issue_type(:test_case).count }
+        end
+      end
+
+      context 'when target namespace has a free plan' do
+        it 'deletes the test cases' do
+          expect { subject.execute(free_group) }.to change { project.issues.with_issue_type(:test_case).count }.from(1).to(0)
+        end
+      end
+    end
   end
 end