selftests/bpf: Move existing common networking parts into network_helpers

1. Move pkt_v4 and pkt_v6 into network_helpers and adjust the users.
2. Copy-paste spin_lock_thread into two tests that use it.

Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Acked-by: Andrey Ignatov <rdna@fb.com>
Link: https://lore.kernel.org/bpf/20200508174611.228805-3-sdf@google.com