As the connection is only established to test authentication, attribute
types are never used and we don't need to retrieve them

This corresponds better what happens for multi-value attributes, where
an empty set is returned

Some multi-value attributes were written although they where never
changed. Accessing them created an empty set, which is different from
the old value None. This unintented writing caused errors, when the user
only has permission to read the attribute.

The test-suite was updated to use a none-admin user for testing.