add "long full recoding" test

this will

• recode for \#steps \in [10, 20, 100]
• at t = 0, k random shards among the n encoded will be selected at random
• at t \geq 1, all k shards will be used to recode k brand new shards
• make sure the last set of k shards recoded \#steps together can decode the data

example with (k, n) = (3, 5) and \#steps = 3

• (s_i)_{1 \leq i \leq k} are the k source shards
• (e_j)_{1 \leq j \leq n} are the n encoded shards
• (m_i)_{1 \leq i \leq k} are the k randomly selected shards
• (n_i)_{1 \leq i \leq k} are the shards after step 1
• (o_i)_{1 \leq i \leq k} are the shards after step 2
• (p_i)_{1 \leq i \leq k} are the shards after step 3
• the (p_i)_{1 \leq i \leq k} will be used for decoding

graph TD;

    s1 --> e1; s1 --> e2; s1 --> e3; s1 --> e4; s1 --> e5;
    s2 --> e1; s2 --> e2; s2 --> e3; s2 --> e4; s2 --> e5;
    s3 --> e1; s3 --> e2; s3 --> e3; s3 --> e4; s3 --> e5;

    e1 --> m1;
    e3 --> m2;
    e4 --> m3;

    m1 --> n1; m1 --> n2; m1 --> n3;
    m2 --> n1; m2 --> n2; m2 --> n3;
    m3 --> n1; m3 --> n2; m3 --> n3;

    n1 --> o1; n1 --> o2; n1 --> o3;
    n2 --> o1; n2 --> o2; n2 --> o3;
    n3 --> o1; n3 --> o2; n3 --> o3;

    o1 --> p1; o1 --> p2; o1 --> p3;
    o2 --> p1; o2 --> p2; o2 --> p3;
    o3 --> p1; o3 --> p2; o3 --> p3;

src/fec.rs

@@ -452,9 +452,12 @@ mod tests {
 
         let mut shards = encode::<F>(data, &Matrix::random(k, n, &mut rng))
             .unwrap_or_else(|_| panic!("could not encode {test_case}"));
+        shards.truncate(k);
 
         for _ in 0..nb_steps {
-            shards.push(recode_random(&shards, &mut rng).unwrap().unwrap());
+            shards = (0..k)
+                .map(|_| recode_random(&shards, &mut rng).unwrap().unwrap())
+                .collect();
         }
 
         try_all_decoding_combinations(data, &shards, k, n, &test_case, Some(100), vec![]);