[win32] fix some unit tests

These commits fix the unit test setup and some unit tests on win32.
* The first commit fixes building and running unit tests from Visual Studio. 
* The second commit silences a warning due to an unused variable (only used by non-win32 platforms)
* The third commit fixes CXBMCTestUtils::ReferenceFilePath() which is used in a lot of places to get a valid path to a test file. It probably worked on non-win32 platforms but was broken on win32 due to putting together paths ending with `\` and starting with `/`.
* The last commit excludes TestRarFile from the unit test project for now because compilation fails due to 47f3d83d3daaab638f6dc52a8ecdeb216733cf99 and I don't really know what the proper fix is. On win32 the missing `S_IFLNK` is never used in any core code only in the unit tests.