[net8.0] Fix the Windows tests (#17939)

1. Keep the .NET 6 feed, because even though we've dropped support for
.NET 6, parts of the build may still need access to the feed.

> Workload installation failed: microsoft.maccatalyst.sdk::16.2.19 is
not found in NuGet feeds [long list of feeds, but not the right one]

2. Pass a runsettings to 'dotnet test' to tell it to use the .NET
version we've downloaded, otherwise it will try to use any .NET version
installed on the system (🤦‍♂️) - which doesn't work because we need to
use .NET 8, and the system only has .NET 7.

NuGet.config

@@ -20,6 +20,8 @@
     <add key="dotnet-public" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet-public/nuget/v3/index.json" protocolVersion="3" />
     <add key="Dotnet arcade" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet-eng/nuget/v3/index.json" />
     <add key="dotnet-tools" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet-tools/nuget/v3/index.json" />
+    <!-- We've dropped support for .NET 6, but let's keep the feed in for a little while longer to make sure it's dropped throughout our entire stack and the lack of this feed doesn't break our build. -->
+    <add key="dotnet6" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet6/nuget/v3/index.json" />
     <add key="dotnet7" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet7/nuget/v3/index.json" />
     <add key="dotnet8" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet8/nuget/v3/index.json" />
     <!-- dotnet8-transport is required for Microsoft.DotNet.Cecil -->

tools/devops/automation/templates/windows/build.yml

@@ -124,6 +124,19 @@ steps:
   displayName: 'Build dependencies for .NET tests'
   continueOnError: true
 
+- pwsh: |
+    $runsettings = @"
+    <?xml version="1.0" encoding="utf-8"?>
+    <RunSettings>
+      <RunConfiguration>
+        <DotNetHostPath>$(Build.SourcesDirectory)\xamarin-macios\tests\dotnet\Windows\bin\dotnet\dotnet.exe</DotNetHostPath>
+      </RunConfiguration>
+    </RunSettings>
+    "@
+    Set-Content -Path $(Build.SourcesDirectory)/xamarin-macios/tests/dotnet/Windows/config.runsettings -Value $runsettings
+    Get-Content -Path $(Build.SourcesDirectory)/xamarin-macios/tests/dotnet/Windows/config.runsettings | Write-Host
+  displayName: 'Create runsettings for .NET tests'
+
 - pwsh: |
     $Env:DOTNET = "$(Build.SourcesDirectory)\xamarin-macios\tests\dotnet\Windows\bin\dotnet\dotnet.exe"
     & $(Build.SourcesDirectory)\xamarin-macios\tests\dotnet\Windows\bin\dotnet\dotnet.exe `
@@ -131,6 +144,7 @@ steps:
         "$(Build.SourcesDirectory)/xamarin-macios/tests/dotnet/UnitTests/DotNetUnitTests.csproj" `
         --filter Category=Windows `
         --verbosity quiet `
+        --settings $(Build.SourcesDirectory)/xamarin-macios/tests/dotnet/Windows/config.runsettings `
         "-bl:$(Build.SourcesDirectory)/xamarin-macios/tests/dotnet/Windows/run-dotnet-tests.binlog"
   displayName: 'Run .NET tests'