Fix handling of skip_permission_denied on windows; after converting
the return value of GetLastError() to a standard error_code, ec.value()
is in the standard errc range, not a native windows error code. This
was missed in 156180727d6c347eda3ba749730707acb8a48093.
The directory "C:\System Volume Information" does seem to exist and
have these properties, but try to verify its existence first.
I guess one could/should factor out the initial setup code to a helper
in support/filesystem_test_helper.h. @curdeius, WDYT?