If there are third-party .m4 files visible to aclocal, that trigger warnings or errors, the "Specific warnings options for autoreconf" test (torture.at:1865) will fail when autoreconf runs aclocal.  For instance:

275. torture.at:1865: testing Specific warnings options for autoreconf ...
./torture.at:1873: automake --version || exit 77
stderr:
stdout:
automake (GNU automake) 1.16.1
Copyright (C) 2018 Free Software Foundation, Inc.
License GPLv2+: GNU GPL version 2 or later <https://gnu.org/licenses/gpl-2.0.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

Written by Tom Tromey <tromey@redhat.com>
       and Alexandre Duret-Lutz <adl@gnu.org>.
./torture.at:1897: autoreconf -Werror -Wall
0a1,5
> aclocal: warnings are treated as errors
> /opt/csw/share/aclocal/gtk.m4:7: warning: underquoted definition of AM_PATH_GTK
> /opt/csw/share/aclocal/gtk.m4:7:   run info Automake 'Extending aclocal'
> /opt/csw/share/aclocal/gtk.m4:7:   or see https://www.gnu.org/software/automake/manual/automake.html#Extending-aclocal
> autoreconf: error: aclocal failed with exit status: 1
./torture.at:1897: exit code was 1, expected 0
275. torture.at:1865: 275. Specific warnings options for autoreconf (torture.at:1865): FAILED (torture.at:1897)

We should make the test suite run aclocal in a way that prevents it from reading third-party .m4 files.

(Originally reported by Dagobert as part of sr #110320, logs extracted from file #49872.)