selftests/bpf: make flow dissector tests more extensible

Rewrite selftest to iterate over an array with input packet and
expected flow_keys. This should make it easier to extend this test
with additional cases without too much boilerplate.

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