selftests/bpf: Allow assigning traffic monitor print function

Allow users to change traffic monitor's print function. If not provided,
traffic monitor will print to stdout by default.

Signed-off-by: Amery Hung <ameryhung@gmail.com>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://patch.msgid.link/20250305182057.2802606-2-ameryhung@gmail.com


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