Bump Microsoft.NET.Test.Sdk from 17.11.1 to 17.12.0 in /samples/snippets/core/testing/unit-testing-using-dotnet-test/csharp in the dotnet group

Bump Microsoft.NET.Test.Sdk from 17.11.1 to 17.12.0 in /samples/snippets/core/testing/unit-testing-using-dotnet-test/csharp in the dotnet group #5396