Support xunit 2 + MTP in Arcade

[Youssef1313]

xunit 2 can run on MTP via YTest.MTP.XUnit2 NuGet package (maintained in https://github.com/Youssef1313/YTest.MTP.XUnit2).

Since lots of repos have issues migrating to xunit.v3 (either technical blockers, or migration being very big and time consuming), I think it might be good to still make a move from VSTest to MTP.

The package above even keeps the usual --filter for MTP, which is even an advantage compared to xunit.v3 which dropped it completely for MTP.

I'm not sure how you prefer this support be gated. A new TestRunnerName, e.g, XUnit2MTP? A new MSBuild property like <ArcadeEnableXunit2MTP>true</ArcadeEnableXunit2MTP>? Something else?

If we got to a point where most repos moved to MTP, we can then later explore making it the default in Arcade (breaking), and slowly work on keeping only MTP support in Arcade.

[Youssef1313]

I'm not sure how you prefer this support be gated. A new TestRunnerName, e.g, XUnit2MTP? A new MSBuild property like true? Something else?

@ViktorHofer @agocke Any preferences here?

[ViktorHofer]

Adding a new TestRunnerName enum makes sense to me.

[mmitche]

Yep agreed.