Here's a diagram of what would(should) happen with ISAPI_Rewrite:
Responses go in reverse, meaning the client always sees the "/testApp/" in the url.
The problem with this is that any links in the html that are returned aren't going to have "/testApp/" in them. You'll either need another IIS filter that handles the rewriting, or some sort of proxy that will rewrite the html.
Another option is to find a content rewriting, reverse proxy, j2ee application that you can mount at /testApp on your tomcat install. I'm not aware of anything that works like that, though.
The last option is to actually change the html generated by your application. Either using a search/and/replace tool to switch all the urls, or by reinstalling it at a different location.