Preliminary results from the buildfarm suggest that no platform gets
commit c6f153dc's test cases wrong by more than one or two units in
the last place, so setting extra_float_digits = 0 should be plenty
to hide the cross-platform variations.

Also, add tests for Infinity/NaN inputs.  I think it highly likely
that we'll end up removing these again, rather than adding code to
make ancient platforms conform.  But it seems useful to find out
just how many platforms have such issues before we make a decision.

Discussion: https://postgr.es/m/E1h3nUY-0000sM-Vf@gemulon.postgresql.org