vim testsuite: Test_keep_pty_open V2

[Zhaoming Luo]

Hi,

This is V2 of figuring out the issue. V1 is abandoned due to my mistake,
but this time I can confirm the source of issue. This time I ran
rpctrace directly on vim, and used two printfs to locate the isatty().

```
printf ("Start isatty!\n");
if (mch_isatty(in))
    channel->ch_to_be_closed |= (1U << PART_IN);
printf ("Finish isatty!\n");
```

rpctrace output:
```
...
30<--44(pid920)->io_write_request ("Start isatty!\n" -1) = 0 14
56<--24(pid920)->term_getctty () = 0x4000002d (Operation not supported)
30<--44(pid920)->io_write_request ("Finish isatty!\n" -1) = 0 15
...
```

The error seems to be from [2].

Best,
Zhaoming

[0]: https://mail.gnu.org/archive/html/bug-hurd/2025-02/msg00009.html
[1]: https://github.com/vim/vim/blob/master/src/channel.c#L1394
[2]: https://git.savannah.gnu.org/cgit/hurd/hurd.git/tree/term/users.c#n362