Ensure authoring test project reports properly on Github Actions

[Mpdreamz]

There are two test engines in .NET core.

• VSTest, the old engine.
• TestPlatform, the brandspanking new platform. Read more here why there is a new one

By default xunit3 will run under TestPlatform however that engine is not allowing --logger arguments so we can not see the github annotations for failed tests.

See: microsoft/testfx#4365 for more background about this missing feature. The author for TSUnit still has a few issues open to properly write extensions to support this.

So here we now run xunit3 under the old VSTest platform so we can run dotnet test as per default with the --logger argument.

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This now also moves both projects over to xunit v3 so both test projects behave the same.

cc @elastic/dotnet