simgrid: compile tests in parallel

pkgs/applications/science/misc/simgrid/default.nix

···
     cat <<EOW >CTestCustom.cmake
     SET(CTEST_CUSTOM_TESTS_IGNORE smpi-replay-multiple)
     EOW
+
+    # make sure tests are built in parallel (this can be long otherwise)
+    make tests -j $NIX_BUILD_CORES
   '';
 
   meta = {