Merge branch 'selftests/bpf: integrate test_tcp_check_syncookie.sh into test_progs'

Alexis Lothoré (eBPF Foundation) says:

====================
this series aims to bring test_tcp_check_syncookie.sh scope into
test_progs to make sure that the corresponding tests are also run
automatically in CI. This script tests for bpf_tcp_{gen,check}_syncookie
and bpf_skc_lookup_tcp, in different contexts (ipv4, v6 or dual, and
with tc and xdp programs).
Some other tests like btf_skc_cls_ingress have some overlapping tests with
test_tcp_check_syncookie.sh, so this series moves the missing bits from
test_tcp_check_syncookie.sh into btf_skc_cls_ingress, which is already
integrated into test_progs.
- the first three commits bring some minor improvements to
  btf_skc_cls_ingress without changing its testing scope
- fourth and fifth commits bring test_tcp_check_syncookie.sh features
  into btf_skc_cls_ingress
- last commit removes test_tcp_check_syncookie.sh

The only topic for which I am not sure for this integration is the
necessity or not to run the tests with different program types:
test_tcp_check_syncookie.sh runs tests with both tc and xdp programs, but
btf_skc_cls_ingress currently tests those helpers only with a tc
program. Would it make sense to also make sure that btf_skc_cls_ingress
is tested with all the programs types supported by those helpers ?

The series has been tested both in CI and in a local x86_64 qemu
environment:
  # ./test_progs -a btf_skc_cls_ingress
  #38/1    btf_skc_cls_ingress/conn_ipv4:OK
  #38/2    btf_skc_cls_ingress/conn_ipv6:OK
  #38/3    btf_skc_cls_ingress/conn_dual:OK
  #38/4    btf_skc_cls_ingress/syncookie_ipv4:OK
  #38/5    btf_skc_cls_ingress/syncookie_ipv6:OK
  #38/6    btf_skc_cls_ingress/syncookie_dual:OK
  #38      btf_skc_cls_ingress:OK
  Summary: 1/6 PASSED, 0 SKIPPED, 0 FAILED

---
Changes in v2:
- fix initial test author mail in Cc
- Fix default cases in switches: indent, action
- remove unneeded initializer
- remove duplicate interface bring-up
- remove unnecessary check and return in bpf program
- Link to v1: https://lore.kernel.org/r/20241016-syncookie-v1-0-3b7a0de12153@bootlin.com
====================

Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>