selftests/net: only print passing message in GRO tests when tests pass

gro.c:main no longer erroneously claims a test passes when running as a
sender.

Tested: Ran `gro.sh -t large` to verify the sender no longer prints a
status.

Signed-off-by: Kevin Krakauer <krakauer@google.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20250226192725.621969-3-krakauer@google.com


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