add flooding with duplicated shards

tests take ~34 on my machine.

src/fec.rs

@@ -305,6 +305,8 @@ mod tests {
             vec![2, 4],       // =  n
             vec![1, 3],       // = (n + 1)
             vec![n, (n + 1)], // = (n + 2) = ((2, 4), (1, 3))
+            vec![0],          // = (n + 3) = (0)
+            vec![0],          // = (n + 4) = (0)
         ];
         let should_not_be_decodable = vec![
             vec![2, 4, n],
@@ -313,6 +315,9 @@ mod tests {
             vec![1, 3, n, (n + 2)],
             vec![2, 4, (n + 1), (n + 2)],
             vec![1, 2, 3, 4, (n + 2)],
+            vec![0, (n + 3)],
+            vec![0, (n + 4)],
+            vec![(n + 3), (n + 4)],
         ];
 
         for step in recoding_steps {