selftests/bpf/test_offload.py: Only check verifier log on verification fails

Since commit 6f8a57cc ("bpf: Make verifier log more relevant by
default"), the verifier discards log messages for successfully-verified
programs. This broke test_offload.py which is looking for a verification
message from the driver callback. Change test_offload.py to use the toggle
in netdevsim to make the verification fail before looking for the
verification message.

Fixes: 6f8a57cc ("bpf: Make verifier log more relevant by default")
Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Jakub Kicinski <kuba@kernel.org>
Link: https://lore.kernel.org/bpf/160752226069.110217.12370824996153348073.stgit@toke.dk