Merge in 'release/3.1' changes

\n\nCommit migrated from dotnet/extensions@9b8667e

Author: dotnet-bot

src/Testing/Directory.Build.props

@@ -4,5 +4,6 @@
   <PropertyGroup>
     <!-- These projects depend on a 3rd party source -->
     <ExcludeFromSourceBuild>true</ExcludeFromSourceBuild>
+    <IsShipping>false</IsShipping>
   </PropertyGroup>
 </Project>

src/Testing/src/Microsoft.AspNetCore.Testing.csproj

@@ -9,7 +9,6 @@
     <!-- This is actually a library for test projects, not a test project. -->
     <IsUnitTestProject>false</IsUnitTestProject>
     <IsPackable>true</IsPackable>
-    <IsShipping>false</IsShipping>
     <HasReferenceAssembly>true</HasReferenceAssembly>
     <GenerateFrameworkReferenceAssembly>true</GenerateFrameworkReferenceAssembly>
     <!-- This package is internal, so we don't generate a package baseline. Always build against the latest dependencies. -->