Indirect the location of cxxtest through a property.

By default CxxTestRoot is ..\..\..\..\ which is what it replaces.  Thus the project should build unchanged on existing machines.

I have made an edit to C:\Users\rdw\AppData\Local\Microsoft\MSBuild\v4.0\Microsoft.Cpp.*.user.props to do the local override.

git-svn-id: https://svn.shibboleth.net/cpp-opensaml/branches/REL_2@664 fb386ef7-a10c-0410-8ebf-fd3f8e989ab0