run FEC decoding test in a loop (!6)

# changelog
- refactor $k$ and $n$ into variables
- run the decoding FEC test in a loop from $(k - 1) \rho + 1$ to $k \rho$, where $\rho$ is the curve number of bytes per scalar field element, e.g. $31$ for _BLS-12-381_
- add a _NOTE_ about having too few bytes and elements, i.e. below $(k - 1) \rho + 1$
- add a _FIXME_ about having too many bytes and elements, i.e. above $k \rho$