[test][freeflow] fix switching between isothermal and nonisothermal model

A preprocessor variable ISOTHERMAL was set to 0 per default in main.cc
while for switching, the variable NONISOTHERMAL was evaluated.
To fix, set NONISOTHERMAL instead of ISOTHERMAL in main.cc.
5 jobs for !2850 with fix/test-freeflow-channel-nonisothermal-switch
latest detached
Status Name Job ID Coverage
  Trigger
passed full-dune-2.7-clang

passed full-dune-2.7-gcc

passed full-dune-master-gcc

passed minimal-dune-2.7-gcc

 
  Downstream Modules
passed trigger lecture