WADL test should use XPath expressions instead of iterating over children

[Leonard Richardson]

The WADL generation test has a bunch of code like this:

    >>> date_targeted = single_list_value(
    ... [param for param in new_milestone_repr
    ... if param.attrib.get('name') == 'date_targeted'])

Rather than iterating over the <param> tags, we should just grab the param tag we want with an XPath expression. The reason we haven't done that traditionally is that the XPath expression takes the fully-qualified name of a tag, which in these tests is huge. But a helper method could avoid that problem.