delete screenshots after uploading

src/Poster.php

@@ -41,6 +41,7 @@ final class Poster{
             $data=$this->repo->addAttachment("/tmp/$name.png");
             $url=$data->browser_download_url;
             $img="\n![]($url)\n";
+            unlink("/tmp/$name.png");
           }
           $this->report.="\n------\n# $name\n$img\n```plain\n".print_r($testcase,true)."\n```";
         }