Create resource using tmpfile() in JSON test

`fopen` fails on Windows with "fopen(.): failed to open stream: Permission denied"

Author: vhenzl

tests/Unit/Drivers/JsonDriverTest.php

@@ -116,7 +116,7 @@ public function it_can_not_serialize_resources()
 
         $this->expectException(CantBeSerialized::class);
 
-        $resource = fopen('.', 'r');
+        $resource = tmpfile();
 
         $driver->serialize($resource);
     }