bug#56025: 29.0.50; em-extpipe-test-2 times out on EMBA and Cygwin

[Eli Zaretskii]

> Cc: larsi@gnus.org, 56025@debbugs.gnu.org
> From: Jim Porter <jporterbugs@gmail.com>
> Date: Fri, 24 Jun 2022 16:03:13 -0700
> 
> POSIX standard says[1] (thanks to Eliot Moss on the Cygwin mailing list 
> for citing this passage):
> 
>    When [EOF is] received, all the bytes waiting to be read are
>    immediately passed to the process without waiting for a <newline>, and
>    the EOF is discarded. Thus, if there are no bytes waiting (that is,
>    the EOF occurred at the beginning of a line), a byte count of zero
>    shall be returned from the read(), representing an end-of-file
>    indication.

Perhaps some reference to this should also be in the comments.