[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
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