Revert "[CMake] Ask for ADDTEST_TIMEOUT only if ADDTEST_MPI_RANKS > 1"

This reverts commit 8bd9ff56.

The commit did not apply the correct fix, as MPI_RANKS is not a number
but a list of ranks instead. Also, I doubt the existence of ">".

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 > 1) AND (NOT ADDTEST_TIMEOUT))
+  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_TIMEOUT)