[CMake] Check for the presence of TIMEOUT before setting the default on MPI_RANKS

cmake/modules/DuneTestMacros.cmake

@@ -196,12 +196,12 @@ function(dune_add_test)
   if(NOT ADDTEST_COMMAND)
     set(ADDTEST_COMMAND ${ADDTEST_NAME})
   endif()
-  if(NOT ADDTEST_MPI_RANKS)
-    set(ADDTEST_MPI_RANKS 1)
-  endif()
   if(ADDTEST_MPI_RANKS 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_MPI_RANKS)
+    set(ADDTEST_MPI_RANKS 1)
+  endif()
   if(NOT ADDTEST_TIMEOUT)
     set(ADDTEST_TIMEOUT 300)
   endif()