test: fix spawn_setgid_fails and spawn_setuid_fails

Don't try to set a bogus UID or GID and expect to get a meaningful
error. The test expected EPERM but SunOS returns EINVAL because the
id is outside of the range of valid user/group ids.

Try to switch to UID/GID 0. Give up privileges first if we're root,
else the setuid/setgid system call will succeed when it's expected
to fail.