selftests/net: have `gro.sh -t` return a correct exit code

Modify gro.sh to return a useful exit code when the -t flag is used. It
formerly returned 0 no matter what.

Tested: Ran `gro.sh -t large` and verified that test failures return 1.
Signed-off-by: Kevin Krakauer <krakauer@google.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20250226192725.621969-2-krakauer@google.com


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