>>Your app. needs to be able to find this DLL. I usually add the DLL to the project (not as a reference, just to the project) then select it and set "Copy to Output Directory" to "Copy always".
>
>Do you know how should I figure out the problems? I've been debugging the test. When I come up to the Invoke1 line I saw a call to Request.Dispose method (request is a separate class that implements IDisposable interface) and then the test ended. So, I assume that this call somehow interrupted the whole thing.

If Request.Dispose() fired unexpectedly then it's a sure sign that your code threw an uncaught exception - which was probably the failure to load the DLL.
You should add your own try/catch block.