Additional information: The application relative virtual path '~/xxx' is not allowed here

Not duplicate since it has a catch.

So As per previous questions I am using ~/ for mapping.So I use:

private static readonly Configuration _config = System.Web.Configuration.WebConfigurationManager.OpenWebConfiguration("~/xxx")

I can also use

private static readonly Configuration _config = System.Web.Configuration.WebConfigurationManager.OpenWebConfiguration(HttpRuntime.AppDomainAppVirtualPath" + /xxx") 
// Which doesn't work as expected

The issue is that web.config is not at the root and I want to read a web.config which is in a sub directory. You might ask why? It's because I'm using Orchard CMS and I only want to read my own module's web.config. This works just fine all the time except for NUnit testing. When I want to test it cannot find my web.config. Now where should I place this file for tests to be able to read it?

And how do I get rid of that error while it works on my tests and the actual application?