Since builtins don't actually have the streams connected, but instead
read input via the io_streams_t objects, this would just always say
what *fish's* fds were.
Instead, pass along some of the stream data to check those
specifically - nobody cares that `test`s fd 0 *technically* is stdin.
What they want to know is that, if they used another program in that
place, it would connect to the TTY.
This is pretty hacky - I abused static variables for this, but
since it's two bools and an int it's probably okay.
See #1228.
Fixes#4766.
