I also am not clear on what you are trying to do and why
<a href="testpage.html">
would not work if testpage is a static page or rewritten URL created within xcart.
I don't use 4.2, but on 4.1.x, you can use a smarty variable that indicates your cart location:
<a href="{$current_location}/testpage.html">
This works in XCART - so if you are making html pages outside xcart, there is more involved in using the smarty variables.