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), ...