Fix analyzer tests to pull packages from our nuget.config feed

This fixes a Azure Pipeline break in the devdiv projedct where network isolation is enabled.

Author: AArnott

test/Microsoft.VisualStudio.Composition.Analyzers.Tests/Helpers/ReferencesHelper.cs

@@ -1,10 +1,32 @@
 // Copyright (c) Microsoft Corporation. All rights reserved.
 // Licensed under the MIT license. See LICENSE file in the project root for full license information.
 
+#pragma warning disable SA1202 // Elements should be ordered by access - because field initializer depend on each other
+
 internal static class ReferencesHelper
 {
+    private static readonly string NuGetConfigPath = FindNuGetConfigPath();
+
     internal static ReferenceAssemblies DefaultReferences = ReferenceAssemblies.NetFramework.Net472.Default
+        .WithNuGetConfigFilePath(NuGetConfigPath)
         .WithPackages(ImmutableArray.Create(
             new PackageIdentity("System.Composition.AttributedModel", "6.0.0"),
             new PackageIdentity("System.ComponentModel.Composition", "6.0.0")));
+
+    private static string FindNuGetConfigPath()
+    {
+        string? path = AppContext.BaseDirectory;
+        while (path is not null)
+        {
+            string candidate = Path.Combine(path, "nuget.config");
+            if (File.Exists(candidate))
+            {
+                return candidate;
+            }
+
+            path = Path.GetDirectoryName(path);
+        }
+
+        throw new InvalidOperationException("Could not find NuGet.config by searching up from " + AppContext.BaseDirectory);
+    }
 }