Fix unit tests

Problem/Motivation

Since we started this module many changes were made, but we couldn't update the unit test for each change.
We need to update the unit test to validate if they still working and fix where need to update.

Steps to reproduce

Make sure to install all drupal core-dev composer packages to execute unit tests.
In a ddev installation execute ddev exec /var/www/html/vendor/bin/phpunit -v -c ./core/phpunit.xml.dist ./modules/contrib/webform_remote_fields/tests
Check the errors and fix all unit tests failed.

Proposed resolution