selftests/bpf: Convert udp_limit test to ASSERT_* macros

Convert the selftest to use the preferred ASSERT_* macros instead of the
deprecated CHECK().

Signed-off-by: Wang Yufen <wangyufen@huawei.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/1664169131-32405-12-git-send-email-wangyufen@huawei.com