ci: add tests GH Action

[alphaolomi]

This pull request adds a new GitHub Actions workflow file tests.yml to the repository. This workflow is triggered on pushes and pull requests to the main branch. It performs the following steps:

1. Validates composer.json and composer.lock using composer validate --strict.
2. Caches Composer packages to speed up future runs.
3. Installs Composer dependencies using composer install.
4. Runs a test suite using the composer run-script test command.

This workflow will help ensure the integrity of the PHP project by running tests whenever changes are made to the main branch.