selftests/bpf: add missing __weak kfunc log fixup test

Add test validating that libbpf correctly poisons and reports __weak
unresolved kfuncs in post-processed verifier log.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20230418002148.3255690-5-andrii@kernel.org


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