selftests/bpf: Switch existing selftests to using open_opts for custom BTF

This patch mainly replaces the bpf_object_load_attr of
the core_autosize.c and core_reloc.c files with bpf_object_open_opts.

Signed-off-by: Shuyi Cheng <chengshuyi@linux.alibaba.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/1626180159-112996-4-git-send-email-chengshuyi@linux.alibaba.com