Improve test and remove the temporary session file created by the skipif

check

tests/basic/bug20539.phpt

@@ -2,6 +2,7 @@
 Bug #20539 (PHP CLI Segmentation Fault)
 --SKIPIF--
 <?php if (!extension_loaded("session")) die("skip session extension not available"); ?>
+<?php unlink(__DIR__. '/sess_' .session_id()); ?>
 --INI--
 session.auto_start=1
 session.save_handler=files
@@ -9,7 +10,10 @@ session.save_path=./tests/basic/
 --FILE--
 <?php
 	print "good :)\n";
-	unlink(dirname(__FILE__) . '/sess_' . session_id());
+	$filename = __DIR__ . '/sess_' . session_id();
+	var_dump(unlink($filename));
 ?>
 --EXPECT--
 good :)
+bool(true)
+