rpc: adjust helper contract code

Compiler forces notification parameters to match the one specified in
manifest, and manifest of `test_contract.go` tells that `to` parameter
type is hash160:
```
event 'Transfer' should have 'Hash160' as type of 1 parameter, got: ByteArray
```

pkg/rpc/server/testdata/test_contract.go

@@ -23,7 +23,7 @@ func Init() bool {
 	h := runtime.GetExecutingScriptHash()
 	amount := totalSupply
 	storage.Put(ctx, h, amount)
-	runtime.Notify("Transfer", []byte{}, h, amount)
+	runtime.Notify("Transfer", interop.Hash160([]byte{}), h, amount)
 	return true
 }