Tests: lower the PHP sleep time in unreleasedXHR.html

The AJAX test performed in unreleasedXHR.html was scheduling PHP processes
sleeping for 10 minutes. When a lot of commits are tested in short intervals
this was causing build failures due to the drained php-fpm pool.

The 10 seconds sleep time should be enough for this test.

Refs 62acda8

test/data/ajax/unreleasedXHR.html

@@ -14,7 +14,7 @@
 	}, 200 );
 	var number = 50;
 	while( number-- ) {
-		jQuery.ajax("../name.php?wait=600");
+		jQuery.ajax("../name.php?wait=10");
 	}
 });
 </script>