Item12888: Fix register test

Fix for buik registration added a more specific message for the case
when a login name is provided but not supported.  Telling the user to
check the LoginNameFilter didn't make sense if the feature is disabled.

git-svn-id: http://svn.foswiki.org/trunk@17778 0b4bb1d4-4e5a-0410-9cc4-b2b747904278