Issue #3413198 by DYdave: Automated testing on GitLab CI: Added initial...

https://www.drupal.org/project/simple_block/issues/3413198

Automated testing on GitLab CI:
Added initial configuration file .gitlab-ci.yml based on template and documentation from Drupal.org documentation: GitLab CI, to provide automated testing with GitLab CI pipelines.

Fixed automated tests issues:

• Fixed eslint and phpcs coding standards errors.
• Fixed phpstan issues by declaring constructors final (see #3412951 and #3405618 for more information)
• Fixed phpunit errors by
  • replacing the classy theme with claro which is now the core admin theme for D10
  • and adding permission to create page content to be able to test a node (simple_block_layout_builder).