[server] fix anonymous_user predicate in tests
devtools' TestServerConfiguration overrides the anonymous_user method,
but not the anonymous-user config option, so testing for the latter
would give the wrong result. Closes #3996664.
[entities] cw_rest_attr_info() should only consider required attributes (closes #3766717)
This prevents CW from choosing unique but non-required attributes. None/NULL is
a poor choice for RESTful URIs.