In Sys..Xml.XPath.XDoc..Tests, add ref to fx libs and fix pkg refs

Add references to System.Text.Encoding 4.0.10 and
System.Text.Encoding.CodePages 4.0.0, as well as make packages.config
consistent with framework references in the project.


Commit migrated from https://github.com/dotnet/corefx/commit/3f88204c9922bdbc2874934f0e49118201439bfc

src/libraries/System.Xml.XPath.XDocument/tests/System.Xml.XPath.XDocument.Tests.csproj

@@ -74,6 +74,17 @@
       <Private>False</Private>
       <SpecificVersion>False</SpecificVersion>
     </Reference>
+    <Reference Include="System.Text.Encoding">
+      <HintPath>..\..\packages\System.Text.Encoding.4.0.10-beta-22405\lib\portable-wpa80+win80+net45+aspnetcore50\System.Text.Encoding.dll</HintPath>
+      <Private>False</Private>
+      <SpecificVersion>False</SpecificVersion>
+    </Reference>
+    <Reference Include="System.Text.Encoding.CodePages">
+      <HintPath>..\..\packages\System.Text.Encoding.CodePages.4.0.0-beta-22405\lib\portable-wpa80+win80+net45+aspnetcore50\System.Text.Encoding.CodePages.dll</HintPath>
+      <Private>False</Private>
+      <SpecificVersion>False</SpecificVersion>
+    </Reference>
+
     <Reference Include="xunit.abstractions">
       <HintPath>..\..\packages\xunit.abstractions.2.0.0-beta5-build2785\lib\net35\xunit.abstractions.dll</HintPath>
     </Reference>
@@ -304,4 +315,4 @@
     <Service Include="{82A7F48D-3B50-4B1E-B82E-3ADA8210C358}" />
   </ItemGroup>
   <Import Project="$(MSBuildExtensionsPath32)\Microsoft\Portable\$(TargetFrameworkVersion)\Microsoft.Portable.CSharp.targets" />
-</Project>
\ No newline at end of file
+</Project>

src/libraries/System.Xml.XPath.XDocument/tests/packages.config

 <?xml version="1.0" encoding="utf-8"?>
 <packages>
+  <package id="System.Runtime" version="4.0.20-beta-22405" />
+  <package id="System.Xml.ReaderWriter" version="4.0.10-beta-22405" />
+  <package id="System.IO" version="4.0.10-beta-22405" />
+  <package id="System.Runtime.Extensions" version="4.0.10-beta-22405" />
+  <package id="System.Reflection" version="4.0.10-beta-22405" />
+  <package id="System.Globalization" version="4.0.10-beta-22405" />
+  <package id="System.Threading.Tasks" version="4.0.10-beta-22405" />
+  <package id="System.Text.Encoding" version="4.0.10-beta-22405" />
+  <package id="System.Text.Encoding.CodePages" version="4.0.0-beta-22405" />
   <package id="xunit" version="2.0.0-beta5-build2785" targetFramework="net45" />
   <package id="xunit.abstractions" version="2.0.0-beta5-build2785" targetFramework="net45" />
   <package id="xunit.assert" version="2.0.0-beta5-build2785" targetFramework="net45" />