Codeception configuration

[Sweetchuck]

Problems

1. Use *.dist.yml file name pattern for Codeception configuration files
2. Namespaces used tests/**/*.php files are not aligned with the namespaces defined in composer.json#/autoload-dev
3. Directory tests/_support/_generated/ should be added to .gitignore.
4. Unnecessary file: phpunit.xml.dist
5. Directory tests/acceptance/fixtures should be somewhere under the tests/_data directory.