added test for touch

As discribed in #37 issueing cberl:touch causes a segfault, running it with lldb I was able
to pinpoint the error to cb.c line 439, what jumps out to me is

```
enif_alloc_binary(cb.ret[i]->nkey, &key_binary);
memcpy(key_binary.data, cb.ret[i]->data, cb.ret[i]->size);
```

which uses nkey for alloc and size for memcpy, changing it did not fix
the error so.