Update test projects to use latest Roslyn.

- This is a temporary workaround until we're able to get on the latest SDK which includes the latest Roslyn bits. Without them we can't properly publish testapps in a standalone fashion without investing in temporary infrastructure.
\n\nCommit migrated from 21fcd9c2f5

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

@@ -77,6 +77,12 @@ namespace Microsoft.AspNetCore.Razor.Design.IntegrationTests
         [InitializeTestProject(originalProjectName: "SimpleMvc", targetProjectName: "Whitespace in name", baseDirectory: "")]
         public async Task Build_AppWithWhitespaceInName_CanBuildSuccessfully()
         {
+            // We need to separately restore the project in order to ensure this project uses the latest Roslyn. Without this
+            // the Csc task from Roslyn would have already been loaded and any update to it from nuget packages would not have
+            // an effect. This allows us to create our obj/Whitespace in name.csproj.nuget.g.props file (we renamed the project)
+            // and then properly build the project with an appropriate Csc.
+            await DotnetMSBuild("Restore");
+
             var result = await DotnetMSBuild(
                 "Build",
                 "/p:_RazorForceBuildServer=true");

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

@@ -38,4 +38,26 @@
 
   <Import Project="After.Directory.Build.props" Condition="Exists('After.Directory.Build.props')" />
 
+  <!--
+    NOTE, BELOW
+
+    These bits are a temporary workaround until we're able to get on the latest SDK which includes the latest Roslyn bits.
+    Without them we can't properly publish testapps in a standalone fashion without investing in temporary infrastructure.
+  -->
+  <PropertyGroup>
+    <MicrosoftNetCompilersToolsetPackageVersion>3.3.0-beta2-19365-06</MicrosoftNetCompilersToolsetPackageVersion>
+    <RestoreSources Condition="'$(DotNetBuildOffline)' != 'true' ">
+      $(RestoreSources);
+      https://dotnet.myget.org/F/roslyn/api/v3/index.json;
+    </RestoreSources>
+  </PropertyGroup>
+  
+  <ItemGroup>
+    <PackageReference Include="Microsoft.Net.Compilers.Toolset"
+        Version="$(MicrosoftNetCompilersToolsetPackageVersion)"
+        PrivateAssets="all"
+        IsImplicitlyDefined="true" />
+  </ItemGroup>
+
+
 </Project>