selftests/bpf: Use ASSERT_* instead of CHECK

Replace usage of CHECK with a corresponding ASSERT_* macro for bpf_iter
tests. Only done if the final result is equivalent, no changes when
replacement means loosing some information, e.g. from formatting string.

Signed-off-by: Dmitrii Dolgov <9erthalion6@gmail.com>
Link: https://lore.kernel.org/r/20220510155233.9815-4-9erthalion6@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>