Move to 3.0 Preview 6 SDK and 2.1.11 runtime (dotnet/aspnetcore-tooling#695)

- add feeds that SDK no longer provides automatically when building test apps
- add another allowed build warning in some integration tests\n\nCommit migrated from cfd4698381

src/Razor/Microsoft.NET.Sdk.Razor/test/IntegrationTests/Assert.cs

@@ -23,6 +23,7 @@ namespace Microsoft.AspNetCore.Razor.Design.IntegrationTests
         private static readonly string[] AllowedBuildWarnings = new[]
         {
             "MSB3491" , // The process cannot access the file. As long as the build succeeds, we're ok.
+            "NETSDK1071", // "A PackageReference to 'Microsoft.NETCore.App' specified a Version ..."
         };
 
         public static void BuildPassed(MSBuildResult result, bool allowWarnings = false)

src/Razor/test/testassets/Directory.Build.props

@@ -11,6 +11,14 @@
 
     <RazorSdkCurrentVersionProps>$(SolutionRoot)src\Microsoft.NET.Sdk.Razor\build\netstandard2.0\Sdk.Razor.CurrentVersion.props</RazorSdkCurrentVersionProps>
     <RazorSdkCurrentVersionTargets>$(SolutionRoot)src\Microsoft.NET.Sdk.Razor\build\netstandard2.0\Sdk.Razor.CurrentVersion.targets</RazorSdkCurrentVersionTargets>
+  
+    <!-- Necessary while testing with unreleased packages. -->
+    <RestoreSources>
+      $(RestoreSources);
+      https://dotnetfeed.blob.core.windows.net/dotnet-core/index.json;
+      https://dotnetfeed.blob.core.windows.net/aspnet-extensions/index.json;
+      https://api.nuget.org/v3/index.json;
+    </RestoreSources>
   </PropertyGroup>
 
   <PropertyGroup>