tests: Fix i386 and powerpc build

Fix:

tests/sys/kern/unix_passfd_test.c:414:24: error: comparison of integers
of different signs: 'int' and 'unsigned int' [-Werror,-Wsign-compare]

ATF_REQUIRE(getnfds() == nfds + MAXFDS);
~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~

powerpc.powerpc/tmp/usr/include/atf-c/macros.h:144:15: note: expanded
from macro 'ATF_REQUIRE'

if (!(expression)) \
      ^~~~~~~~~~

1 error generated.

• unix_passfd_test.o ---