RPC binding config may be malformed or the source .go contract may contain
structures like this:
```
type Str struct {
Field int
field int
}
```
We need to recognise these cases and return error. otherwise the resulting
binding can't be compiled.
Signed-off-by: Anna Shaleva <shaleva.ann@nspcc.ru>
It helps us to keep the index clean and ordered. When some new RPC binding
test should be added, we don't need to search for a suitable place for it.
Signed-off-by: Anna Shaleva <shaleva.ann@nspcc.ru>
