Bump MSTest from 3.6.1 to 3.6.4 in /docs/core/tutorials/snippets/library-with-visual-studio/csharp in the dotnet group across 1 directory

Bump MSTest from 3.6.1 to 3.6.4 in /docs/core/tutorials/snippets/library-with-visual-studio/csharp in the dotnet group across 1 directory #1221