borg2: fix archived mode and mode-related tests (windows)

https://docs.python.org/3/library/stat.html

on linux, there is `stat_result.st_mode` and borg archives / restores that (as expected).

on native windows, this seems to be an emulated value, derived from other information, so there isn't much point in archiving that, but borg could maybe archive `.st_file_attributes`.

also, borg should set these file attributes at restore time.