[bugfix] [Tests] Test coverage for PHP class completion for backend model; bugfix for #126

[vasilii-b]

Description (*)

This PR

1. covers with tests the PHP class completion for backend_model implemented in Completion. Add PHP classes completion to the backend_model attribute and tag #89
2. refactor the test provided in 92 tests on php class completion for the source model #122 to match the new approach discussed with @VitaliyBoyko
3. fix the bug reported in [bug] PHP classes completion to the backend_model attribute and tag not in the right files #126 and spotted during running the tests added in scope of this PR

Fixed Issues (if relevant)

1. [bug] PHP classes completion to the backend_model attribute and tag not in the right files #126: PHP classes completion to the backend_model attribute and tag not in the right files
2. Tests. Cover the "Add PHP classes completion to the backend_model attribute and tag" by tests #124: Tests. Cover the "Add PHP classes completion to the backend_model attribute and tag" by tests

Questions or comments

Contribution checklist (*)

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages