selftests/bpf: Add test for libbpf's custom log_buf behavior

Add a selftest that validates that per-program and per-object log_buf
overrides work as expected. Also test same logic for low-level
bpf_prog_load() and bpf_btf_load() APIs.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20211209193840.1248570-11-andrii@kernel.org