Tests for a project are kept under the tests
folder, which is a peer of the src
folder. If you need to have multiple test projects for a component, structure them in sub folders.
For example, lay things out like this:
tests\
test_project_1\
test_1.cs
test_2.cs
test_project_1.csproj
test_project_2\
test_1.cs
test_2.cs
test_project_2.csproj
Not like this:
tests\
test_project_1.csproj
test_project_2.csproj
test_folder_1\
test_1.cs
test_2.cs
test_folder_2\
test_1.cs
test_2.cs