* test: cover delete-on-close on the windows mount
Windows software creates temporaries with FILE_FLAG_DELETE_ON_CLOSE and
never deletes them explicitly. The conformance suite showed a file
outliving its last handle — an aborted test left its file behind and
every later test hit a name collision — but nothing in this suite asks
for the flag, because os offers no way to.
Skips where the flag is unavailable rather than passing quietly.
* test: fail delete-on-close on a real error instead of skipping
Skipping on any error meant a refused flag looked the same as a platform
that cannot ask for it, so the test could pass by never running. It now
skips only on that one sentinel and reports everything else.
Also stops printing a nil error when the file is still there after its
last handle closed, and checks the closes it was discarding.
