Just echo the javascript out inside the if function
<form name="testForm" id="testForm" method="POST" >
<input type="submit" name="btn" value="submit" autofocus onclick="return true;"/>
</form>
<?php
if(isset($_POST['btn'])){
echo "
<script type=\"text/javascript\">
var e = document.getElementById('testForm'); e.action='test.php'; e.submit();
</script>
";
}
?>