selftests/bpf: add flow dissector bpf_skb_load_bytes helper test

When flow dissector is called without skb, we want to make sure
bpf_skb_load_bytes invocations return error. Add small test which tries
to read single byte from a packet.

bpf_skb_load_bytes should always fail under BPF_PROG_TEST_RUN because
it was converted to the skb-less mode.

Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>