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>

Author: bastien-curutchet

tools/testing/selftests/bpf/Makefile

@@ -95,7 +95,7 @@ TEST_GEN_PROGS += test_progs-cpuv4
 TEST_INST_SUBDIRS += cpuv4
 endif
 
-TEST_GEN_FILES = test_lwt_ip_encap.bpf.o test_tc_edt.bpf.o
+TEST_GEN_FILES = test_tc_edt.bpf.o
 TEST_FILES = xsk_prereqs.sh $(wildcard progs/btf_dump_test_case_*.c)
 
 # Order correspond to 'make run_tests' order
@@ -104,7 +104,6 @@ TEST_PROGS := test_kmod.sh \
 	test_lirc_mode2.sh \
 	test_xdp_vlan_mode_generic.sh \
 	test_xdp_vlan_mode_native.sh \
-	test_lwt_ip_encap.sh \
 	test_tc_tunnel.sh \
 	test_tc_edt.sh \
 	test_xdping.sh \