Give the 3D reacting mixing layer an explicit equation of state

The case set pi_inf but left eos unset, so the test harness's BASE_CFG default of ideal_gas survived the merge and validation rejected the pair. The 2D case already states stiffened_gas alongside the same zero pi_inf.

Author: sbryngelson

examples/3D_reacting_mixing_layer/case.py

@@ -161,6 +161,7 @@
     "prim_vars_wrt": "T",
     "parallel_io": "T",
     "fluid_pp(1)%gamma": 1.0 / (fluid["gamma"] - 1.0),
+    "fluid_pp(1)%eos": "stiffened_gas",
     "fluid_pp(1)%pi_inf": 0.0,
     "fluid_pp(1)%Re(1)": 1.0 / fluid["viscosity"],
     "patch_icpp(1)%geometry": 9,