test: refactor test-tls-alert-handling

* process.on('exit',...) checks -> common.mustCall()
* remove unused function parameters
* var -> const/let

PR-URL: https://github.com/nodejs/node/pull/10482


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Italo A. Casas <me@italoacasas.com>