selftests/bpf: Add test to cover ktls with bpf_msg_pop_data

selftests/bpf: Add test to cover ktls with bpf_msg_pop_data

The selftest can reproduce an issue where using bpf_msg_pop_data() in
ktls causes errors on the receiving end.

Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/20250609020910.397930-3-jiayuan.chen@linux.dev