working on FEC tests again

• pass n to try_all_decoding_combinations and don't try to decode when shards have been recoded (\#shards > n) and there are no recoded shards in the k combination under review (\max(is) < n)
• pass recoding_steps and should_not_be_decodable as arguments to end_to_end_with_recoding_template
• fix n = 5 => this leads to tests that run in less than 10sec again
• add (k, n) = (8, 10) => tests still run in less than 13sec
• split recoding scenarii into "simple" and "chain"
• show indices in a "pretty" format, i.e. showing indices greater than n as (n), (n + 1), ...

src/fec.rs

@@ -426,7 +426,7 @@ mod tests {
             ]
         }
 
-        for (k, n) in [(3, 5), (5, 5)] {
+        for (k, n) in [(3, 5), (5, 5), (8, 10)] {
             for (name, steps, should_not_decode) in get_scenarii(n) {
                 end_to_end_with_recoding_template::<Fr>(
                     &bytes,