test: refactor beforeExit tests

Combine and rename tests for the `beforeExit` event on `process`.

The naming now more closely follows the de facto conventions of the
project.

The two tests were very similar and do not seem to benefit from being
separate.

PR-URL: https://github.com/nodejs/node/pull/10581


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>