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: #11367
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>

test/parallel/test-tls-cert-chains-in-ca.js

@@ -12,8 +12,7 @@ const {
 
 // agent6-cert.pem includes cert for agent6 and ca3, split it apart and
 // provide ca3 in the .ca property.
-const agent6Chain = keys.agent6.cert.split('-----END CERTIFICATE-----')
-  .map((c) => { return c + '-----END CERTIFICATE-----'; });
+const agent6Chain = keys.agent6.cert.split(/(?=-----BEGIN CERTIFICATE-----)/);
 const agent6End = agent6Chain[0];
 const agent6Middle = agent6Chain[1];
 connect({