test: add regression test for 512 bits DH key

Check that trying to use a < 1024 bits DH key throws an exception.

parallel/test-tls-dhe tests this as well but it feels incongruous not to
do it here when both tests have similar logic for 1024/2048 bits keys.

PR-URL: https://github.com/nodejs/node/pull/3629
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>

test/parallel/test-tls-client-mindhsize.js

@@ -75,6 +75,9 @@ function testDHE2048() {
 
 testDHE1024();
 
+assert.throws(() => test(512, true, assert.fail),
+              /DH parameter is less than 1024 bits/);
+
 process.on('exit', function() {
   assert.equal(nsuccess, 1);
   assert.equal(nerror, 1);