Merge in 'release/6.0' changes

Author: dotnet-bot

src/Mvc/Mvc.Testing/src/Microsoft.AspNetCore.Mvc.Testing.targets

@@ -46,7 +46,7 @@
     </ItemGroup>
   </Target>
 
-  <Target Name="_GeneratePublishTestManifest" BeforeTargets="Publish" DependsOnTargets="_ResolveMvcTestProjectReferences">
+  <Target Name="_GeneratePublishTestManifest" Condition="$(IsPublishable) == 'true'" BeforeTargets="Publish" DependsOnTargets="_ResolveMvcTestProjectReferences">
     <ItemGroup>
       <_PublishManifestProjects Include="%(_ContentRootProjectReferences.FusionName)">
         <ContentRoot>~</ContentRoot>