test: refactor parallel/test-tls-ca-concat.js

PR-URL: https://github.com/nodejs/node/pull/19092
Fixes: https://github.com/nodejs/node/issues/14544


Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: James M Snell <jasnell@gmail.com>