working on FEC tests again (dragoon/komodo!146)

- 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)`, ...