test: refactor test-tls-securepair-fiftharg

Assert the server name directly in the `SNICallback`,
since `common.mustCall()` already guarantees that the callback
is called exactly once, making `process.on('exit')` unnecessary.

PR-URL: https://github.com/nodejs/node/pull/17836


Reviewed-By: Timothy Gu <timothygu99@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Jon Moss <me@jonathanmoss.me>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>