Add gitlab.ci for testing on CI.

Problem/Motivation

Our project requires a robust CI/CD pipeline to support our Drupal development workflow. Currently, we lack automated testing infrastructure, making it difficult to maintain code quality and detect regressions early in the development cycle.

Steps to reproduce

1. Clone the repository
2. Note the absence of GitLab CI configuration
3. Observe that all testing must be performed manually
4. Recognize the inefficiency of this approach for team development

Proposed resolution

Implement a comprehensive GitLab CI/CD pipeline with appropriate test coverage:

• Create a .gitlab-ci.yml file with build, lint, test, and deploy stages
• Configure PHPUnit for automated testing
• Implement code standards validation
• Set up deployment to testing environments

Remaining tasks

1. Create initial .gitlab-ci.yml configuration
2. Set up Docker containers for testing environments
3. Implement PHPUnit test suite for custom module functionality
4. Configure code standards checks (PHPCS with Drupal standard)
5. Set up test reporting and notifications
6. Document the CI/CD workflow for the development team

User interface changes

None. This is an infrastructure improvement.

API changes

None. This ticket focuses on testing infrastructure only.

Data model changes

None. No data model modifications are required for this task.