Bump MSTest.TestFramework from 3.4.1 to 3.5.1 in /docs/core/tutorials/snippets/library-with-visual-studio/vb/StringLibraryTest

Bump MSTest.TestFramework from 3.4.1 to 3.5.1 in /docs/core/tutorials/snippets/library-with-visual-studio/vb/StringLibraryTest #1514