test: fix warning in dlopen-ping-pong/binding.cc

Currently, the following compiler warning is issued:
../binding.cc:26:27:
warning: 'Utf8Value' is deprecated [-Wdeprecated-declarations]
  const String::Utf8Value filename(args[0]);
                          ^

This commit updates the code to use the Isolate version.

PR-URL: https://github.com/nodejs/node/pull/19966


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>