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: #19966
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

Author: danbev

test/addons/dlopen-ping-pong/binding.cc

@@ -23,7 +23,7 @@ typedef const char* (*ping)(void);
 static ping ping_func;
 
 void LoadLibrary(const FunctionCallbackInfo<Value>& args) {
-  const String::Utf8Value filename(args[0]);
+  const String::Utf8Value filename(args.GetIsolate(), args[0]);
   void* handle = dlopen(*filename, RTLD_LAZY);
   assert(handle != nullptr);
   ping_func = reinterpret_cast<ping>(dlsym(handle, "dlopen_ping"));