Fix fulltest TIOCGPGRP failure when run from shell

Fix fulltest TIOCGPGRP failure when run from shell

When fulltest runs from /bin/sh (rather than as init directly), the
shell has already claimed the controlling TTY via setsid(), setting
tty_fg_pgrp to a non-zero value. The test assumed fg == 0, which
only holds when running as PID 1 (session_id=0, pgrp_id=0).

Fix: compare TIOCGPGRP result against sys_getpgrp() instead of
hardcoded 0, use our own pgrp for TIOCSPGRP success test, and use
fg=-1 (EINVAL) instead of fg=1 for the expected-failure case.

Also rename 'hello from init.elf' to 'hello from fulltest.elf' in
both the test binary and the smoke test expectations.