XPath: Select self and following sibling together

If it must be a single XPath 1.0 expression then you'll have to say

//dt[contains(., 'Test')] | //dt[contains(., 'Test')]/following-sibling::dd[1]

The final [1] is important, as without that it would extract all dd elements that follow a dt containing "Test", i.e. given

<div>
    <dt>
        Test 1
    </dt>
    <dd>
        Foo
    </dd>
    <dt>
        Something else 2
    </dt>
    <dd>
        Bar
    </dd>
</div>

the version without the [1] would match three nodes, the dt containing "Test 1" and both the "Foo" and "Bar" dd elements. With the [1] you would correctly get only "Test 1" and "Foo".

But depending on exactly how you're using the XPath it may be clearer to first select

//dt[contains(., 'Test')]

and then iterate over the nodes that this matches, and evaluate

. | following-sibling::dd[1]

in the context of each of those nodes in turn.

When using XPath 2.0:

//dt[contains(text(), "Test")]/(self::dt, following-sibling::dd)