Move XBUILD_FRAMEWORK_FOLDERS_PATH definition somewhere useful & export

This variable isn't being passed to the child xbuild process during testing, making it sorta useless.
Use net_4_5 libraries for xbuild tests, regardless of target.

This works around failures caused by tests asking for other frameworks (like 4.0) on Mono 4.0, which only has 4.5
Merge pull request #1456 from directhex/fix_Microsoft.Build.Execution.BuildManagerTest.BuildCommonResolveAssemblyReferences

Fix Microsoft.Build.Execution.BuildManagerTest.BuildCommonResolveAssemblyReferences