Cannot change username with SSO enabled

[andrei (Andrei Prigorshnev)]

This PR adds test cases for the problem described in the OP, so we can be sure that in general overriding username, for example, from jb1 to jb works:

DEV: add test cases that prove that sso can override username from "bob1" to "bob"

@fefrei

I found it: sso overrides username prevents the name change.

I still consider this situation a bug. Enabling this setting should either hide the Change Username button from the administrator panel and disable the users/.../preferences/username page, or still allow username changes for admins (possibly with a warning).

I’m going to take a look at it.