[2.x] Test collection issue

Problem/Motivation

This issue is dedicated to collect remaining tests, that should be implemented for this module.

• Add a test for testing the correct error message, when logging in with incorrect mail-address/password. Our custom validation error should throw ("Unrecognized email address or password [...]"), instead of the core message.
• https://www.drupal.org/project/email_registration/issues/3396368 ✨ Add Username Enumeration Attack Prevention Test Active
• ?

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes