selftests/bpf: Bring test_offload.py back to life

Bpftool has new extra libbpf_det_bind probing map we need to exclude.
Also skip trying to load netdevsim modules if it's already loaded (builtin).

v2:
- drop iproute2->bpftool changes (Toke)

Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20221206232739.2504890-1-sdf@google.com