Re: Why does the build change the startpage of my project to ClientBin/TestPage.html
By default, when you create a hosting website, it sets the hosting site's default start page to the test page. If you open the Web App's properties (either double-click the Properties (C#) or My Project (VB) folder in Solution Explorer, or right-click the project and select Properties); from there select the Web tab, and you'll see that the Start Action is set to the specific test page.
I'm not sure why your setting isn't being saved, I'll look into it and see if this is expected or not.
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SDET, Microsoft Web Developer team