selftests/bpf: Add test for the use of new args in cong_control

This patch adds a selftest to show the usage of the new arguments in
cong_control. For simplicity's sake, the testing example reuses cubic's
kernel functions.

Signed-off-by: Miao Xu <miaxu@meta.com>
Link: https://lore.kernel.org/r/20240502042318.801932-4-miaxu@meta.com


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