`ethexe`: test the case when code is comming for validation but has been already processed

```
            // TODO: test this case
```

In file ethexe/compute/src/lib.rs
https://github.com/gear-tech/gear/blob/203853ff3c75ee5d3f2211514f313bf2a5d3400c/ethexe/compute/src/lib.rs#L176