tests: fix build failure on gcc-6.3 / glibc-2.25 (open)

[trofi]

Recent gcc has a mechanism to detect some function misuses
at compile-time. In this case it's a case when open() is
likely to create new file but access permissions are not set
explicitly:

$  LANG=C make -C tests api-tests
gcc -g2 -O2 -Wall -fno-omit-frame-pointer -DHAVE_CONFIG_H  -DLINUX  -I.. -I../include   -c -o api/fsio.o api/fsio.c
In file included from /usr/include/fcntl.h:313:0,
                 from /usr/include/sys/file.h:24,
                 from ../include/conf.h:93,
                 from api/tests.h:30,
                 from api/fsio.c:27:
In function 'open',
    inlined from 'fsio_sys_access_file_test' at api/fsio.c:1059:6:
/usr/include/bits/fcntl2.h:50:4: error: call to '__open_missing_mode' declared with attribute error: open with O_CREAT or O_TMPFILE in second argument needs 3 arguments
    __open_missing_mode ();
    ^~~~~~~~~~~~~~~~~~~~~~
make: *** [<builtin>: api/fsio.o] Error 1

The change adds "u=rw" to all temp files.

Signed-off-by: Sergei Trofimovich slyfox@gentoo.org

[coveralls]

Coverage remained the same at 81.634% when pulling 1f2b190 on trofi:master into 0d97c3e on proftpd:master.

[Castaglia]

Nice. Thanks!