I had the same problem. You can have a situation when Properties will not help you to copy app.config where you need it. Such as when you do integration unit testing. Then you can have two choices:
Use post build event
copy /Y “$(ProjectDir)App.config” “....\test\Debug\$(TargetFileName).config”
Manually copy config file sections from libA.dll to libA.Test.dll
I end up using the 2 as it proved to be more robust for me.