selftests/net: Print the testing side in unsigned-md5

As both client and server print the same test name on failure or pass,
add "[server]" so that it's more obvious from a log which side printed
"ok" or "not ok".

Signed-off-by: Dmitry Safonov <0x7f454c46@gmail.com>
Link: https://patch.msgid.link/20250319-tcp-ao-selftests-polling-v2-5-da48040153d1@gmail.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>