Cover the module with node tests

Currently, there are no tests in the module. We should create some tests to check the base features.
I guess steps should be smth like:
- create a regular user with permission to create nodes
- create a new content type with a text field
- add configs - the text field should be unique
- submit the node form with the text field = test
- the node should be created without any errors
- submit the node form with the text field = test
- the user should have an error message
- add a 'unique_field_perm_bypass' permission to the user
- submit the node form with the text field = test
- the node should be created without any errors

p.s. .gitlab-ci.yml file should be updated to execute tests.