selftests/bpf/xdp_redirect_multi: Put the logs to tmp folder

The xdp_redirect_multi test logs are created in selftest folder and not cleaned
after test. Let's creat a tmp dir and remove the logs after testing.

Fixes: d2329247 ("selftests/bpf: Add xdp_redirect_multi test")
Suggested-by: Jiri Benc <jbenc@redhat.com>
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20211027033553.962413-2-liuhangbin@gmail.com