seltests: bpf: test_tunnel: Use ip neigh

The 'arp' command is deprecated and is another dependency of the selftest.
Just use 'ip neigh', the test depends on iproute2 already.

Signed-off-by: Jiri Benc <jbenc@redhat.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/40f24b9d3f0f53b5c44471b452f9a11f4d13b7af.1632236133.git.jbenc@redhat.com