suzerain: add check_ideal_gas_approximation

Runs on test_subsonic for now.  test_supersonic was not
created with this requirement in mind.  It will now be
updated to match.

git-svn-id: https://svn.ices.utexas.edu/repos/pecos/suzerain/trunk@42711 0bd85f0b-c151-43b9-a1e9-27231104551f