selftests/bpf: expand bpf tunnel test with decap

The bpf tunnel test encapsulates using bpf, then decapsulates using
a standard tunnel device to verify correctness.

Once encap is verified, also test decap, by replacing the tunnel
device on decap with another bpf program.

Signed-off-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>