Added conditional skipIf to test_external_modules, darwin machines don't have tcl modules so skip this test and let user know why
Added conditional skipIf to test_external_modules, darwin machines don't have tcl modules so skip this test and let user know why

EmreAtes/spack,LLNL/spack,skosukhin/spack,matthiasdiener/spack,TheTimmy/spack,EmreAtes/spack,krafczyk/spack,iulian787/spack,tmerrick1/spack,EmreAtes/spack,krafczyk/spack,tmerrick1/spack,skosukhin/spack,tmerrick1/spack,LLNL/spack,LLNL/spack,iulian787/spack,TheTimmy/spack,matthiasdiener/spack,lgarren/spack,skosukhin/spack,lgarren/spack,krafczyk/spack,mfherbst/spack,krafczyk/spack,iulian787/spack,skosukhin/spack,mfherbst/spack,EmreAtes/spack,EmreAtes/spack,TheTimmy/spack,mfherbst/spack,TheTimmy/spack,tmerrick1/spack,iulian787/spack,TheTimmy/spack,krafczyk/spack,skosukhin/spack,LLNL/spack,iulian787/spack,tmerrick1/spack,matthiasdiener/spack,matthiasdiener/spack,lgarren/spack,matthiasdiener/spack,mfherbst/spack,lgarren/spack,LLNL/spack,mfherbst/spack,lgarren/spack