Fix System.Xml.XmlDocument.Tests build on 32-bit Windows

Build currently fails on 32-bit Windows with the following error:

    C:\Program Files\MSBuild\Microsoft\VisualStudio\v12.0\AppxPackage\Microsoft.AppXPackage.Targets(657,9): 
    error APPX0502: File 'C:\ Program Files (x86)\Windows Kits\8.1\bin\x86\makepri.exe' not found. [C:\Work\
    GitHub\dotnet\corefx\src\System.Xml.XmlDocument\tes ts\System.Xml.XmlDocument.Tests.csproj]