Fix build on 32-bit Windows

Like PR #109, there are 4 more projects failing to build on 32-bit Windows 7

- System.Xml.XDocument.Tests.csproj
- System.Xml.XPath.XDocument.Tests.csproj
- System.Xml.XPath.XmlDocument.Tests.csproj
- System.Xml.XPath.Tests.csproj