test: use newish OpenSSL feature in test-tls-dhe

According to the comment that is being replaced here, this was not
possible with the current version of OpenSSL 1.0.2 at the time the test
was written. New OpenSSL versions appear to always print the length of
the temporary key.

Refs: https://github.com/nodejs/node-v0.x-archive/pull/8272
PR-URL: https://github.com/nodejs/node/pull/46708
Reviewed-By: Filip Skokan <panva.ip@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

test/parallel/test-tls-dhe.js

@@ -77,8 +77,8 @@ function test(keylen, expectedCipher, cb) {
       out += d;
     });
     client.stdout.on('end', function() {
-      // DHE key length can be checked -brief option in s_client but it
-      // is only supported in openssl 1.0.2 so we cannot check it.
+      assert(keylen === 'error' ||
+             out.includes(`Server Temp Key: DH, ${keylen} bits`));
       const reg = new RegExp(`Cipher    : ${expectedCipher}`);
       if (reg.test(out)) {
         nsuccess++;