test: add regex for expected error message

Provide a regex to validate the error message.

PR-URL: nodejs/node#12011
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>

test/parallel/test-tls-key-mismatch.js

@@ -8,6 +8,8 @@ if (!common.hasCrypto) {
 const assert = require('assert');
 const tls = require('tls');
 const fs = require('fs');
+const errorMessageRegex = new RegExp('^Error: error:0B080074:x509 ' +
+  'certificate routines:X509_check_private_key:key values mismatch$');
 
 const options = {
   key: fs.readFileSync(common.fixturesDir + '/keys/agent1-key.pem'),
@@ -16,4 +18,4 @@ const options = {
 
 assert.throws(function() {
   tls.createSecureContext(options);
-});
+}, errorMessageRegex);