`unittest` placement in code?

[Vladimirs Nordholm via Digitalmars-d-learn]

Hello.

I have a class which I have written some tests for, to ensure if 
I ever change some code it will still work as intended.

The documentation https://dlang.org/spec/unittest.html says it is 
can be placed both in the class or outside it.

I come from a background of having a completely separate tests 
folder with only tests, so I do not know what the general 
best-practice is with D. Where should the `unittest` code block 
go?