The following TestContainer was not found code example

Example: The following TestContainer was not found

The problem was that the NuGet package 
Microsoft.NET.Test.Sdk was not installed. 
Installing this package in the project via the NuGet Package Manager 
solved the problem.

This wasn't intuitive to me since I have another unit test project that 
runs fine without the Microsoft.NET.Test.Sdk package, 
but that project is .NET Framework 4.6.2.