Consider moving to MTP

[Youssef1313]

Currently, dotnet/msbuild is run using VSTest. It's better to move to Microsoft.Testing.Platform.

This can be done by:

1. Moving to xunit.v3+MTP
2. Moving to MSTest+MTP

It looks like originally dotnet/msbuild was MSTest but was migrated to xunit for reasons that don't apply today. #6

I don't know if you want to consider moving to MSTest now.

[Evangelink]

I'd love to be able to weigh-in and suggest migration to MSTest as the team and product are really active! You would get access to new features, dedicated support and obviously partner priorization on feature requests.

[rainersigwald]

We actually use xUnit v2 today.

I am not opposed to moving and prefer to dogfood .NET projects, but we're not currently hitting a huge amount of friction with our test runner so have limited budget to change it.

If we could get better test parallelization in Arcade's test protocol in our PR builds, though, that'd potentially be very big--right now each project serializes testing the .NET Framework and .NET TF outputs, which can be a bottleneck.