test-open: on GNU/Hurd, /dev/null is a directory

test-open: on GNU/Hurd, /dev/null is a directory

* tests/test-fopen.h (main): Rename...
(test_fopen): ...to this.  Use a guaranteed non-directory when
confirming open behavior on trailing slash.
* tests/test-openat-safer.c (main): Likewise.
* tests/test-open.h (main): Likewise....
(test_open): ...to this.
* tests/test-fopen.c (main): Adjust caller.
* tests/test-fopen-safer.c (main): Likewise.
* tests/test-open.c (main): Likewise.
* tests/test-fcntl-safer.c (main): Likewise.
Reported by Samuel Thibault.

Signed-off-by: Eric Blake <ebb9@byu.net>