complete the FEC and "linear algebra" tests

• komodo::linalg::Matrix::random is tested
• komodo::linalg::Matrix::inverse is tested on more matrix sizes, from
  1
  to
  20
  random matrices
• komodo::field tests have been double-checked
• pure "recoding" tests from komodo::fec have been double-checked
• end_to_end and end_to_end_with_recoding now runs for
  k \in [3, 5]
  and
  \rho \in [\frac{1}{2}, \frac{1}{3}]
  with
  n = \lfloor \frac{k}{\rho} \rfloor
• all "
  k
  among
  n + t
  " combinations are tested with try_all_decoding_combinations, possibly with some removals in case recoding is involved with is_inside

Important
on my machine, make test goes from less than 8sec on latest main to around 40sec with this MR