Remove the obsolete register_user helper.

User registrations are now handled by the PendingUser class & LDAP
interactions via libnemesis, so this isn't needed.

Change-Id: If07045b21d714dc7509393d9b3843bb1482191b3
Move duplicate setUp method to helpers, rename.

This also avoids it being run just because it happens to have the
right name.

Change-Id: Ibd6da32505a4e25dcbe03faa32ab6eb018c1e638