[2.x] Create tests testing all of the username validations

Problem/Motivation

There are currently only some generic tests implemented, to verify this module's installation / uninstallation process. We should add functional tests for all validations applied to both the "user_form" and "user_register_form".
These tests should include:
Username contains ...

• Blacklisted string
• Blacklisted char
• Spaces
• Too many characters
• Not enough characters

Steps to reproduce

Proposed resolution

Create proposed tests.

Remaining tasks

User interface changes

API changes

Data model changes