selftests/bpf: Add more tests for test_txmsg_push_pop in test_sockmap

Add more tests for test_txmsg_push_pop in test_sockmap for better coverage

Signed-off-by: Zijian Zhang <zijianzhang@bytedance.com>
Reviewed-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/r/20241106222520.527076-6-zijianzhang@bytedance.com


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