selftests: adopt BPF's approach to quieter builds

selftest build is fairly noisy, it's easy to miss warnings.
It's standard practice to add alternative messages in
the Makefile. I was grepping for existing solutions,
and found that bpf already has the right knobs.

Move them to lib.mk and adopt in net.
Convert the basic rules in lib.mk.

Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/r/20240411190534.444918-1-kuba@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>