Visual Studio 15.8.1 not running MS unit tests

I had to add the following Nuget packages:

MSTest.TestAdapter
MSTest.TestFramework
Microsoft.NET.Test.Sdk

Visual Studio release notes

Ok, you can add Nuget packages as asked. But you can also try to disable the following setting (Tools->Options->Test): "For improved performance, only use test adapters in test assembly folder or as specified in runsettings file".

Let me know if it works for you.