core: test file writes for home dir perms

Creating directories for some reason was allowed in Windows, but writes
would fail later.
core: be sure to close tempfile handle so Windows can delete it
synced_folders/rsync: run hostpath through fs_real_path

This fixes issues with drive letter casing on Windows
core: add test for fs_real_path on Windows
core: environment tests passing on Windows
provider/virtualbox: fix NFS tests on Windows
provisioners/docker: fix wrong variable usage

caught by @pda