.net 5 support

[egil]

Adds multi target framework support for bUnit.

PR meta checklist

• Pull request is targeting at DEV branch.
• Pull request is linked to all related issues, if any.
• I have read the CONTRIBUTING.md document.

Content checklist

• My code follows the code style of this project and AspNetCore coding guidelines.
• My change requires a change to the documentation.
  • I have updated the documentation accordingly.
• I have updated the appropriate sub section in the CHANGELOG.md.
• I have added, updated or removed tests to according to my changes.
  • All tests passed.
• Set up build that target both net5.0 and netstandard2.1, to allow users still on earlier versions of Blazor to get the new features