selftests/bpf: Move test_lwt_ip_encap to test_progs

selftests/bpf: Move test_lwt_ip_encap to test_progs

test_lwt_ip_encap.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_ip_encap.sh. It uses the same network topology and the same BPF
programs located in progs/test_lwt_ip_encap.c.
Rework the GSO part to avoid using nc and dd.

Remove test_lwt_ip_encap.sh and its Makefile entry.

Signed-off-by: Bastien Curutchet (eBPF Foundation) <bastien.curutchet@bootlin.com>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://patch.msgid.link/20250304-lwt_ip-v1-1-8fdeb9e79a56@bootlin.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>