test: add coverage for client._addHandle()

`Client.prototype._addHandle()` in the `_debugger` module has conditions
around invalid properties that are not currently tested. This change
adds some minimal unit tests.

PR-URL: https://github.com/nodejs/node/pull/8518


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>