Add `install_test` for `nest` package

Minor changes to add an install test to the NEST package. This can be used by site-admins to test the basic functionality of the package with the standard "spack test" call.

import nest starts a nest kernel including possibly opening MPI communicators, etc. Errors in environment setup would likely show up here. The nest.get() fetches the kernel status dictionary to see if the kernel is alive and well.