Bump Microsoft.NET.Test.Sdk from 17.8.0 to 18.8.1 (#8)

---
updated-dependencies:
- dependency-name: Microsoft.NET.Test.Sdk
  dependency-version: 18.8.1
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: Arnar Freyr Kristinsson <arnarfkr@gmail.com>

Author: dependabot[bot]