[VFS] Update unittest to fix Windows buildbot.

Buildbot http://lab.llvm.org:8011/builders/clang-x64-windows-msvc is failing
because it doesn't like paths in VFS, make them more Windows-friendly.

Follow up to r347009.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@347016 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed