Fix NET_4_0 tests build by correcting accessibility of two properties that
should be public according to MSDN, but were internal:

http://msdn.microsoft.com/en-us/library/microsoft.build.evaluation.project.targets(v=vs.100).aspx
http://msdn.microsoft.com/en-us/library/microsoft.build.execution.projectinstance.targets(v=vs.100).aspx