test: check that pending warning is emitted once

Code for the new --pending-deprecation flag contains logic to make sure
the deprecation warning is emitted only once. However, this was not
being tested. Add test coverage for this situation.

PR-URL: https://github.com/nodejs/node/pull/12527


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>