SimpleTest: assertFieldByXPath() doesn't work in some environments

This is a follow-up from #295719: node.test cleanup and #299186: SimpleTest: assertFieldByXPath does not work for selects or textareas with value. When these two were committed, I got two failing tests in the node module.

I tracked this down to assertFieldByXPath() that doesn't work properly with textareas. However, it seems that this is related to the PHP version or perhaps the version of libxml installed on the system.

Consider this very simple example:

<?php
$xml = new SimpleXMLElement('<foo>abc</foo>');
foreach ($xml->xpath('//foo') as $bar) {
    var_dump(isset($bar[0]));
}

On PHP 5.2.0-8+etch11 (Debian) with libxml 2.6.27, it outputs bool(false), but on another system running PHP 5.2.2 (Fedora 7) with libxml 2.6.28 outputs bool(true).

The attached patch appears to fix the problem on my Debian system (I cannot easily test it on the Fedora system). However, my experience with SimpleXML is limited.