Merge branch 'selftests-bpf-migrate-test_flow_dissector-sh-to-test_progs'

Alexis Lothoré says:

====================
This is the revision 3 of test_flow_dissector_migration.sh into
test_progs. This revision addresses comments from Stanislas, especially
about proper reuse of pseudo-header checksuming in new network helpers.

There are 2 "main" parts in test_flow_dissector.sh:
- a set of tests checking flow_dissector programs attachment to either
  root namespace or non-root namespace
- dissection test

The first set is integrated in flow_dissector.c, which already contains
some existing tests for flow_dissector programs. This series uses the
opportunity to update a bit this file (use new assert, re-split tests,
etc)
The second part is migrated into a new file under test_progs,
flow_dissector_classification.c. It uses the same eBPF programs as
flow_dissector.c, but the difference is rather about how those program
are executed:
- flow_dissector.c manually runs programs with BPF_PROG_RUN
- flow_dissector_classification.c sends real packets to be dissected, and
  so it also executes kernel code related to eBPF flow dissector (eg:
__skb_flow_bpf_to_target)

---
Changes in v3:
- Keep new helpers name in sync with kernel ones
- Document some existing network helpers
- Properly reuse pseudo-header csum helper in transport layer csum
  helper
- Drop duplicate assert
- Use const for test structure in the migrated test
- Simplify shutdown callchain for basic test
- collect Acked-by
- Link to v2: https://lore.kernel.org/r/20241114-flow_dissector-v2-0-ee4a3be3de65@bootlin.com

Changes in v2:
- allow tests to run in parallel
- move some generic helpers to network_helpers.h
- define proper function for ASSERT_MEMEQ
- fetch acked-by tags
- Link to v1: https://lore.kernel.org/r/20241113-flow_dissector-v1-0-27c4df0592dc@bootlin.com
====================

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