Bump MSTest (#43333)

Bumps the dotnet group in /docs/core/tutorials/snippets/library-with-visual-studio/csharp/StringLibraryTest with 1 update: [MSTest](https://github.com/microsoft/testfx).


Updates `MSTest` from 3.6.1 to 3.6.2
- [Release notes](https://github.com/microsoft/testfx/releases)
- [Changelog](https://github.com/microsoft/testfx/blob/main/docs/Changelog.md)
- [Commits](microsoft/testfx@v3.6.1...v3.6.2)

---
updated-dependencies:
- dependency-name: MSTest
  dependency-type: direct:production
  update-type: version-update:semver-patch
  dependency-group: dotnet
...

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]