Changes

smdpp_Tests: rename smdp_server_url to smdp_server_fqdn

The member name "smdp_server_url" is technically not correct as the
content is an FQDN. Let's rename it to "smdp_server_fqdn".

Let's also put a comment in which we explain why FQDN is hardcoded
and not available as configurable module parameter.

Related: SYS#7339
Change-Id: I0224c838a368656821fff6014b0df7eb49466b0a