'Verify that `all` is allowed in packages.yaml.'

def test_all_is_not_a_virtual(self):

    conf = syaml.load('all:\n                                variants:    [+mpi]\n')
    spack.config.update_config('packages', conf, 'concretize')
    spack.package_prefs.PackagePrefs.clear_caches()
    spack.package_prefs.get_packages_config()
