Permit version mismatches during package testing for incremented ref assms

We're walking the closure of packages that build on top of netcoreapp2.1. Those build using live references but the current package testing infrastructure tests against an LKG framework package. As a result when we bump reference versions every package will see a break until we get an updated LKG with the new reference assemblies.

Once we have a new Microsoft.NETCore.App we can remove this closure exception.

Author: ericstj

pkg/test/frameworkSettings/netcoreapp2.1/settings.targets

@@ -6,4 +6,12 @@
     <!-- use the most recent MS.NETCore.App we have from upstack -->
     <RuntimeFrameworkVersion>$(MicrosoftNETCoreAppPackageVersion)</RuntimeFrameworkVersion>
   </PropertyGroup>
+
+  <ItemGroup>
+    <!-- Temporarily suppress checking closure of Memory and Threading.Tasks.Extensions
+         These wil fail until we get an updated Microsoft.NETCore.App.
+         https://github.com/dotnet/corefx/issues/29249 -->
+    <IgnoredReference Include="System.Memory" />
+    <IgnoredReference Include="System.Threading.Tasks.Extensions" />
+  </ItemGroup>
 </Project>