suzerain: remove stupid Ma^-2 in energy flux

Wow.  There's no 1/Ma/Ma factor in the energy equation
in conserved flux form.  This much better explains the
lousy tolerances I was seeing.  Tightens tolerances.
Moves to relative tests for euler_conservative.

git-svn-id: https://svn.ices.utexas.edu/repos/pecos/suzerain/trunk@42737 0bd85f0b-c151-43b9-a1e9-27231104551f