selftests/bpf: add CO-RE relocs ptr-as-array tests

Add test validating correct relocation handling for cases where pointer
to something is used as an array. E.g.:

int *ptr = ...;
int x = ptr[42];

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