It depends upon how you structure your code base. Basically what I do is mirror all my projects/namespaces/functions so each one has a test project associated with it (even if it is empty for the time being). To organize my files I mirror the namespaces within the solution, consider the namespace (with company being the namespace root):

company.library.project

would be the physical path:

Solution\Library\Project.csproj

To manage my tests I would then have a folder in the root level of the solution called ‘Tests’ which would mirror the tests for each library, however I would name my test library for project

company.library.project.tests

which would have the physical path:

Solution\Tests\Library\Project.csproj

This is of course entirely up to you, you may wish to place your test projects side by side with your projects.

Hope that helps.