[tests] fix duplicate sources in NuGet.config (#8772)

Context: NuGet/Home#13269 (comment)

In 2f4e01e, we saw a problem with no-op NuGet restore, no longer
being a no-op. So we temporarily disabled the failing
`IncrementalFastDeployment` test.

It turns out, it might be related to a duplicate source listed in our
`NuGet.config` during MSBuild tests:

    <add key="dotnet8" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet8/nuget/v3/index.json" />
    <add key="testsource2" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet8/nuget/v3/index.json" />

This might have surfaced a bug in NuGet, but for now let's stop
emitting duplicate sources?

Additionally, use `ExtraNuGetConfigSources.Distinct ()` in case we have a test
that passes in duplicate sources.

src/Xamarin.Android.Build.Tasks/Tests/Xamarin.ProjectTools/Common/XamarinProject.cs

@@ -395,7 +395,6 @@ public void CopyNuGetConfig (string relativeDirectory)
 
 		/// <summary>
 		/// Updates a NuGet.config based on sources in ExtraNuGetConfigSources
-		/// If target framework is not the latest or default, sources are added for previous releases
 		/// </summary>
 		protected void AddNuGetConfigSources (string nugetConfigPath)
 		{
@@ -415,14 +414,8 @@ protected void AddNuGetConfigSources (string nugetConfigPath)
 				ExtraNuGetConfigSources = new List<string> ();
 			}
 
-			if (TargetFramework?.IndexOf ("net8.0", StringComparison.OrdinalIgnoreCase) != -1
-				|| TargetFrameworks?.IndexOf ("net8.0", StringComparison.OrdinalIgnoreCase) != -1) {
-				ExtraNuGetConfigSources.Add ("https://api.nuget.org/v3/index.json");
-				ExtraNuGetConfigSources.Add ("https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet8/nuget/v3/index.json");
-			}
-
 			int sourceIndex = 0;
-			foreach (var source in ExtraNuGetConfigSources) {
+			foreach (var source in ExtraNuGetConfigSources.Distinct ()) {
 				var sourceElement = new XElement ("add");
 				sourceElement.SetAttributeValue ("key", $"testsource{++sourceIndex}");
 				sourceElement.SetAttributeValue ("value", source);

tests/MSBuildDeviceIntegration/Tests/InstallTests.cs

@@ -495,7 +495,6 @@ public void LocalizedAssemblies_ShouldBeFastDeployed ()
 		[Test]
 		public void IncrementalFastDeployment ()
 		{
-			Assert.Ignore ("https://github.com/NuGet/Home/issues/13269");
 			AssertCommercialBuild ();
 
 			var class1src = new BuildItem.Source ("Class1.cs") {