Don't publish Microsoft.AspNetCore.Testing (#4059)

Addresses dotnet/dnceng#174
Relates to #4057

(cherry picked from commit 163e293)

Author: RussKie

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

@@ -6,6 +6,12 @@
     <PackageTags>$(PackageTags);Testing</PackageTags>
   </PropertyGroup>
 
+  <!-- See https://github.com/dotnet/extensions/issues/4057 -->
+  <PropertyGroup>
+    <IsPackable>false</IsPackable>
+    <IsShipping>false</IsShipping>
+  </PropertyGroup>
+
   <PropertyGroup>
     <TargetFrameworks>$(NetCoreTargetFrameworks)</TargetFrameworks>
     <InjectExperimentalAttributeOnLegacy>true</InjectExperimentalAttributeOnLegacy>