Add unit tests for Helix images

This would validate that Helix is correctly configured in these images. There are currently no tests for the images produced by this repo (the Test stage is skipped in the pipeline). But the infrastructure supports this so it's just a matter of defining the tests. And now we do have a place to put them too: https://github.com/dotnet/dotnet-buildtools-prereqs-docker/tree/main/tests/Microsoft.DotNet.BuildTools.Prereqs.Docker.Tests