Add tests for roles

Problem/Motivation

We've had a series of bugs relating to roles, many of them caused by bad fixes to previous bugs.

Proposed resolution

Add tests for as many as possible of these cases:

• 🐛 Cannot remove roles when editing users Fixed caused by #3311553: Non-visible roles are removed →
• #3172576: Admin user cannot bulk add/remove roles when using VBO →
• #3292173: Roles no longer visible on user_form since 3.1.0 release → caused by #3188812: Module overrides previously set #access on roles form element →

Remaining tasks

User interface changes

API changes

Data model changes