test: refactor test-tls-cert-chains-in-ca

When splitting PEM string into separate certs, use non-capturing regexp
to avoid having to put the split string back with .map(). As a bonus,
this splits the PEM into two certs, rather than 2 certs and a third
crufty whitespace-only string.

PR-URL: https://github.com/nodejs/node/pull/11367


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>