spec: support a test prefix
spec: support a test prefix

This is used in tests to test real packages without actually building
and installing them.

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