Ensure that tests are optional (#3922)

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

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

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

var/spack/repos/builtin/packages/hdf5-blosc/package.py

var/spack/repos/builtin/packages/hdf5/package.py

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

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

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

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

...spack/repos/builtin/packages/planck-likelihood/package.py

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

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

var/spack/repos/builtin/packages/py-yt/package.py