selftests/bpf: lwt_seg6local: Move test to test_progs

test_lwt_seg6local.sh isn't used by the BPF CI.

Add a new file in the test_progs framework to migrate the tests done by
test_lwt_seg6local.sh. It uses the same network topology and the same BPF
programs located in progs/test_lwt_seg6local.c.
Use the network helpers instead of `nc` to exchange the final packet.

Remove test_lwt_seg6local.sh and its Makefile entry.

Signed-off-by: Bastien Curutchet (eBPF Foundation) <bastien.curutchet@bootlin.com>
Link: https://lore.kernel.org/r/20250307-seg6local-v1-2-990fff8f180d@bootlin.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>