rpc/client: add constant to ValidUntilBlock

There are 2 problems:
1. `getvalidators` RPC can return empty list.
2. `+1` in single node can be too resrictive.
Proper solution for (1) may require requesting
standby validators. Here we add constant
to fix occasional test failures.

pkg/rpc/client/rpc.go

@@ -489,7 +489,7 @@ func (c *Client) CalculateValidUntilBlock() (uint32, error) {
 			expiresAt:       blockCount + cacheTimeout,
 		}
 	}
-	return blockCount + validatorsCount, nil
+	return blockCount + validatorsCount + 1, nil
 }
 
 // AddNetworkFee adds network fee for each witness script and optional extra