[CMake] Ask for ADDTEST_TIMEOUT only if ADDTEST_MPI_RANKS > 1

ADDTEST_MPI_RANKS is set two lines above if it has been emtpy.

cmake/modules/DuneTestMacros.cmake

@@ -199,7 +199,7 @@ function(dune_add_test)
   if(NOT ADDTEST_MPI_RANKS)
     set(ADDTEST_MPI_RANKS 1)
   endif()
-  if(ADDTEST_MPI_RANKS AND (NOT ADDTEST_TIMEOUT))
+  if((ADDTEST_MPI_RANKS > 1) AND (NOT ADDTEST_TIMEOUT))
     message(FATAL_ERROR "dune_add_test: You need to specify the TIMEOUT parameter if using the MPI_RANKS parameter.")
   endif()
   if(NOT ADDTEST_TIMEOUT)