test: refactor exitedAfterDisconnect test

* minor refactoring to conform with test-writing layout guidelines
* rename test to use current terminology rather than deprecated
  terminology
* assert.strictEqual() -> assert.ok()

PR-URL: https://github.com/nodejs/node/pull/16729


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Eugene Ostroukhov <eostroukhov@google.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>