suzerain: use ideal EOS in residual check

git-svn-id: https://svn.ices.utexas.edu/repos/pecos/suzerain/trunk@42716 0bd85f0b-c151-43b9-a1e9-27231104551f
suzerain: remove misleading radial_euler test

git-svn-id: https://svn.ices.utexas.edu/repos/pecos/suzerain/trunk@42717 0bd85f0b-c151-43b9-a1e9-27231104551f