test-fd-util: compare FDs to /bin/sh instead of /dev/null

test-fd-util: compare FDs to /bin/sh instead of /dev/null

/dev/null is a character device, so same_fd() in the fallback path
that compares fstat will fail, as that bails out if the fd refers
to a char device. This happens on kernels without F_DUPFD_QUERY and
without kcmp.

/* test_same_fd */
Assertion 'same_fd(d, e) > 0' failed at src/test/test-fd-util.c:111, function test_same_fd(). Aborting.

Fixes #35552