WIP infrastructure for Spack test command to test existing installations

lib/spack/spack/build_environment.py

lib/spack/spack/cmd/test.py

lib/spack/spack/cmd/unit_test.py

lib/spack/spack/package.py

lib/spack/spack/test/cmd/test.py

lib/spack/spack/util/executable.py

var/spack/repos/builtin/packages/python/package.py