[convmixing][test] check only if compiles, as we have different results on different machines

lecture/mm/convectivemixing/CMakeLists.txt

 add_input_file_links()
 
-dune_add_test(NAME convmixexercise
+dune_add_test(COMPILE_ONLY # the test produces different results due to the fingers on different machines
+              NAME convmixexercise
               SOURCES convmixexercise.cc
               COMMAND ${dumux_INCLUDE_DIRS}/bin/testing/runtest.py
               CMD_ARGS  --script fuzzy