One of our NuGet audit alerts was rooted in our xUnit packages as they
only had `netstandard1.3` libraries. To resolve this alert I moved us to
newer versions of the package which had `netstandard2.0` libraries.

Upgrading the package revealed two problems in our code base:

1. The xUnit `Assert` APIs took advantage of the `unmanaged` constraint
   in a fwe places. That is not supported by Visual Basic and leads to
   errors on import. To work around this I inserted `AssertEx` thunks
   that our Visual Basic layer can call through. Longer term we will
   need to consider adding support for the constraint
   ([issue](dotnet#75063))
2. The xUnit APIs significantly increased their nullable reference type
   annotations. This caught a number of places where the tests were
   passing potentially null values to APIs that don't accept null. There
   were enough hits that I added `AssertEx` methods that handle the
   nullable case. In a few places though I just suppressed the warning.

The vulnerability: GHSA-cmhx-cq75-c4mj