Right approach to create Test projects

I've got a quite huge WPF solution (46 projects right now), divided in projects for the WPF part and the Web one. I wish to introduce coverage test in my solution (and blame me for not having done it before).

I wish to know what's the best approach to do so, since I don't want to put the Test files inside the projects (I think production code should only contains production code) and at the same time I don't want to add other 46 projects to the solution.

I tought to create another solution where I add those test and each project but doing so I've to sync manually the 2 solution..

Anyone has managed in a clear way a similar scenario?