Ensure a couple more assemblies are available

- don't seem to be copied into test folder in CI builds

Author: dougbu

test/System.Net.Http.Formatting.Test/System.Net.Http.Formatting.Test.csproj

@@ -32,7 +32,7 @@
     <Reference Include="System.Memory, Version=4.0.1.2, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51, processorArchitecture=MSIL">
       <HintPath>..\..\packages\System.Memory.4.5.5\lib\netstandard2.0\System.Memory.dll</HintPath>
       <SpecificVersion>False</SpecificVersion>
-      <Private>False</Private>
+      <Private>True</Private>
     </Reference>
     <Reference Include="System.Net.Http" />
     <Reference Include="System.Net.Http.WebRequest" />
@@ -45,7 +45,7 @@
     <Reference Include="System.Threading.Tasks.Extensions, Version=4.2.0.1, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51, processorArchitecture=MSIL">
       <HintPath>..\..\packages\System.Threading.Tasks.Extensions.4.5.4\lib\portable-net45+win8+wp8+wpa81\System.Threading.Tasks.Extensions.dll</HintPath>
       <SpecificVersion>False</SpecificVersion>
-      <Private>False</Private>
+      <Private>True</Private>
     </Reference>
     <Reference Include="System.XML" />
     <Reference Include="System.Xml.Linq" />