Add new tests for RoleDelegationSettingsForm

Problem/Motivation

The current tests use the "administer users" and "administer permissions" and then change roles via /user/%/edit which doesn't actually test the /user/%/roles page at all.

Proposed resolution

Add new tests to ensure it works.
Abstract out the role building logic from addRoleDelegationElement() so we can test it in isolation rather than through the form.

Remaining tasks

Code/review

User interface changes

API changes

Data model changes