selftests/bpf: add simple BPF_PROG_TEST_RUN examples for flow dissector

Use existing pkt_v4 and pkt_v6 to make sure flow_keys are what we want.

Also, add new bpf_flow_load routine (and flow_dissector_load.h header)
that loads bpf_flow.o program and does all required setup.

Signed-off-by: Stanislav Fomichev <sdf@google.com>
Acked-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>