test: add test for cluster.worker.destroy()

Add a simple test to cover workers' implementation of
Worker.prototype.destroy(). Before adding this test, this code wouldn't
be covered by the tests suite, and any regression introduced in workers'
implementation of Worker.prototype.destroy wouldn't be caught.

Fixes: https://github.com/joyent/node/issues/8223
Reviewed-by: Trevor Norris <trev.norris@gmail.com>