Enable "IsWindowsOnlyTest"s on the Windows.10.Arm64.Open Helix queue

[halter73]

#26840 (re)disabled "IsWindowsOnlyTest"s on the Windows.10.Arm64.Open Helix queue because a bunch of work items failed without explicit test failures. At least some of these work item failures were caused by an error in TestMatrix.cs during theory discovery.

[xUnit.net 00:00:01.16] IIS.NewShim.FunctionalTests: Exception thrown during theory discovery on 'Microsoft.AspNetCore.Server.IIS.FunctionalTests.StartupTests.HelloWorld'; falling back to single test case.
System.ArgumentException: Arm64
   at Microsoft.AspNetCore.Server.IntegrationTesting.TestMatrix.ResolveDefaultArchitecture() in /_/src/Hosting/Server.IntegrationTesting/src/TestMatrix.cs:line 134
   at Microsoft.AspNetCore.Server.IntegrationTesting.TestMatrix.Build() in /_/src/Hosting/Server.IntegrationTesting/src/TestMatrix.cs:line 101
   at Microsoft.AspNetCore.Server.IntegrationTesting.TestMatrix.GetEnumerator()+MoveNext() in /_/src/Hosting/Server.IntegrationTesting/src/TestMatrix.cs:line 327
   at System.Linq.Enumerable.SelectEnumerableIterator`2.MoveNext()

We should properly enable these skipped tests on the Windows.10.Arm64.Open Helix queue. We could do this incrementally by removing the $(IsWindowsOnlyTest)' != 'true' check https://github.com/dotnet/aspnetcore/pull/26840/files#diff-fecf6f98b5c2a5cc5c66f045a93d30421b7fedcbdd4cacefc2605cd61a9b89afR46 and then putting <SkipHelixArm>true</SkipHelixArm> in the still-failing test projects.

We've moved this issue to the Backlog milestone. This means that it is not going to be worked on for the coming release. We will reassess the backlog following the current release and consider this item at that time. To learn more about our issue management process and to have better expectation regarding different types of issues you can read our Triage Process.

[Tratcher]

This initial error is just because of a missing case in a switch:

aspnetcore/src/Hosting/Server.IntegrationTesting/src/TestMatrix.cs

Lines 127 to 134 in fab9e87

	case Architecture.X86:
	Architectures.Add(RuntimeArchitecture.x86);
	break;
	case Architecture.X64:
	Architectures.Add(RuntimeArchitecture.x64);
	break;
	default:
	throw new ArgumentException(RuntimeInformation.OSArchitecture.ToString());

No telling what we'll hit after fixing this though.