test: change indexOf to includes in ecdh-disable

Updates test/parallel/test-tls-ecdh-disable.js on line 34 so that
assert.notEqual is changed to just assert. Then in place of indexOf,
includes() is used. The callback on line 31 has been wrapped in
common.mustCall() in order to ensure that this callback is only
made one time.

test/parallel/test-tls-ecdh-disable.js

@@ -28,10 +28,10 @@ server.listen(0, '127.0.0.1', function() {
   if (common.isWindows)
     cmd += ' -no_rand_screen';
 
-  exec(cmd, function(err, stdout, stderr) {
+  exec(cmd, common.mustCall(function(err, stdout, stderr) {
     // Old versions of openssl will still exit with 0 so we
     // can't just check if err is not null.
-    assert.notEqual(stderr.indexOf('handshake failure'), -1);
+    assert(stderr.includes('handshake failure'));
     server.close();
-  });
+  }));
 });