builtin test: Let -t work for the standard streams

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.

tests/pexpects/isatty.py

@@ -0,0 +1,59 @@
+#!/usr/bin/env python3
+from pexpect_helper import SpawnedProc
+import subprocess
+import sys
+import time
+
+sp = SpawnedProc()
+send, sendline, sleep, expect_prompt, expect_re, expect_str = (
+    sp.send,
+    sp.sendline,
+    sp.sleep,
+    sp.expect_prompt,
+    sp.expect_re,
+    sp.expect_str,
+)
+expect_prompt()
+
+sendline("test -t 0; echo $status")
+expect_prompt("0")
+
+sendline("""function t
+test -t 0 && echo stdin
+test -t 1 && echo stdout
+test -t 2 && echo stderr
+end""")
+expect_prompt()
+
+sendline("t")
+expect_str("stdin")
+expect_str("stdout")
+expect_str("stderr")
+expect_prompt()
+
+sendline("cat </dev/null | t")
+expect_str("stdout")
+expect_str("stderr")
+expect_prompt()
+
+sendline("t | cat")
+expect_str("stdin")
+expect_str("stderr")
+expect_prompt()
+
+sendline("t 2>| cat")
+expect_str("stdin")
+expect_str("stdout")
+expect_prompt()
+
+sendline("cat </dev/null | t | cat")
+expect_str("stderr")
+expect_prompt()
+sendline("cat </dev/null | t 2>| cat")
+expect_str("stdout")
+expect_prompt()
+
+sendline("t </dev/null")
+expect_str("stdout")
+expect_str("stderr")
+expect_prompt()