Add a CI/CD testing guide to the dev docs

This page documents the unofficial CI/CD integration specs.

doc/development/cicd/_index.md

@@ -59,6 +59,8 @@ Development guides that are specific to CI/CD are listed here:
 - If you are adding a new keyword or changing the CI schema, refer to the following guides:
   - [The CI configuration guide](configuration.md)
   - [The CI schema guide](schema.md)
+- If you are making a change to core CI/CD process such as linting or pipeline creation, refer to the
+    [CI/CD testing guide](testing.md)
 
 See the [CI/CD YAML reference documentation guide](cicd_reference_documentation_guide.md)
 to learn how to update the [CI/CD YAML syntax reference page](../../ci/yaml/_index.md).

doc/development/cicd/testing.md

+---
+stage: Verify
+group: Pipeline Authoring
+info: Any user with at least the Maintainer role can merge updates to this content. For details, see https://docs.gitlab.com/ee/development/development_processes.html#development-guidelines-review.
+title: Testing guide for CI/CD Rails application code
+---
+
+This document contains details for testing CI/CD application code.
+
+## Backend
+
+### Integration specs
+
+The CI/CD specs include informal integration specs for the core CI/CD processes.
+
+#### Linting
+
+Integration specs for linting are kept in `spec/lib/gitlab/ci/yaml_processor_spec.rb` and
+`spec/lib/gitlab/ci/yaml_processor/test_cases/`. Add any new specs to the
+`test_cases/` directory.
+
+#### Pipeline creation
+
+Integration specs for pipeline creation are kept in `spec/services/ci/create_pipeline_service_spec.rb` and
+`spec/services/ci/create_pipeline_service/`. Add new specs to the
+`create_pipeline_service/` directory.
+
+#### Pipeline processing
+
+`spec/services/ci/pipeline_processing/atomic_processing_service_spec.rb` runs integration specs for pipeline processing.
+To add a new integration spec, add a YAML CI/CD configuration file to `spec/services/ci/pipeline_processing/test_cases`.
+It is run automatically with `atomic_processing_service_spec.rb`.
+
+## Frontend
+
+### Fixtures
+
+The following files contain frontend fixtures for CI/CD endpoints used in frontend unit tests:
+
+- `spec/frontend/fixtures/pipelines.rb` - General pipeline fixtures
+- `spec/frontend/fixtures/pipeline_create.rb` - Pipeline creation fixtures
+- `spec/frontend/fixtures/pipeline_details.rb` - Pipeline details fixtures
+- `spec/frontend/fixtures/pipeline_header.rb` - Pipeline header fixtures
+- `spec/frontend/fixtures/pipeline_schedules.rb` - Pipeline schedule fixtures
+
+These fixtures provide mock API responses for consistent testing of CI/CD frontend components.
+
+### Unit tests
+
+Frontend unit tests for CI/CD components are located in spec/frontend/ci. These tests verify proper rendering, interactions, and state management for pipeline visualization, job execution, scheduling, and status reporting components.