Fix switching between isothermal and nonisothermal model in freeflow channel test

Merged Bernd Flemisch requested to merge fix/test-freeflow-channel-nonisothermal-switch into master

A preprocessor variable ISOTHERMAL was set to 0 per default in while for switching, the variable NONISOTHERMAL was evaluated. To fix, set NONISOTHERMAL instead of ISOTHERMAL in

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