Bump Microsoft.NET.Test.Sdk from 17.10.0 to 17.11.0 in /docs/core/tutorials/snippets/library-with-visual-studio/csharp/StringLibraryTestNet8 in the dotnet group

Bump Microsoft.NET.Test.Sdk from 17.10.0 to 17.11.0 in /docs/core/tutorials/snippets/library-with-visual-studio/csharp/StringLibraryTestNet8 in the dotnet group #3321