selftests/bpf: add selftests for wide loads

Mirror existing wide store tests with wide loads. The only significant
difference is expected error string.

Cc: Yonghong Song <yhs@fb.com>
Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>