selftests/bpf: add CO-RE relocs testing setup

Add CO-RE relocation test runner. Add one simple test validating that
libbpf's logic for searching for kernel image and loading BTF out of it
works.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Acked-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>