I stumbled upon the same problem. It seems to me that I have to install more and more with each release.
In my case (Visual Studio 2019 community edition, version 16.6.1), I now also had to obtain the Microsoft.NET.Test.Sdk via NuGet.
So in the end, I have installed three packages in my test project:
- NUnit (3.12.0)
- NUnit3TestAdapter (3.17.0)
- Microsoft.NET.Test.Sdk (16.7.1)
Turning the automated test discovery on or off did not change anything for me though.
Hope this helps anyone.