zdtm: add a test to check O_TMPFILE

man 2 open:
...
O_TMPFILE (since Linux 3.11)

Create  an unnamed temporary file.  The pathname argument speci‐ fies a
directory; an unnamed  inode  will  be  created  in  that directory's
filesystem.  Anything written to the resulting file will be lost when
the last file descriptor is closed, unless the file is given a name.
...

Reviewed-by: Dmitry Safonov <0x7f454c46@gmail.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>