Bump Microsoft.TestPlatform.ObjectModel in /tests/NLog.UnitTests (#4318)
Bump Microsoft.TestPlatform.ObjectModel in /tests/NLog.UnitTests (#4318)

Bumps [Microsoft.TestPlatform.ObjectModel](https://github.com/microsoft/vstest) from 16.8.3 to 16.9.1.
- [Release notes](https://github.com/microsoft/vstest/releases)
- [Commits](https://github.com/microsoft/vstest/compare/v16.8.3...v16.9.1)

Signed-off-by: dependabot-preview[bot] <5bdcd3c0d4d24ae3e71b3b452a024c6324c7e4bb@dependabot.com>

Co-authored-by: dependabot-preview[bot] <6a4c1c4838f800d1998274cd5234e1f65c55e90c@users.noreply.github.com>
304NotModified/NLog,snakefoot/NLog,NLog/NLog