tests: fold: check that all files are processed

* tests/fold/multiple-files.sh: New file.
* tests/local.mk (all_tests): Add the test.

tests/fold/multiple-files.sh

@@ -0,0 +1,36 @@
+#!/bin/sh
+# Test fold with multiple files.
+
+# Copyright (C) 2026 Free Software Foundation, Inc.
+
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+. "${srcdir=.}/tests/init.sh"; path_prepend_ ./src
+print_ver_ fold
+
+# Test that all files are processed.
+echo a > file1
+echo b > file2
+returns_ 1 fold file1 missing file2 > out 2> err || fail=1
+cat <<EOF > exp-out || framework_failure_
+a
+b
+EOF
+cat <<EOF > exp-err || framework_failure_
+fold: missing: No such file or directory
+EOF
+compare exp-out out || fail=1
+compare exp-err err || fail=1
+
+Exit $fail

tests/local.mk

@@ -349,6 +349,7 @@ all_tests =					\
   tests/fold/fold-spaces.sh			\
   tests/fold/fold-zero-width.sh			\
   tests/fold/fold.pl				\
+  tests/fold/multiple-files.sh			\
   tests/groups/groups-dash.sh			\
   tests/groups/groups-process-all.sh		\
   tests/groups/groups-version.sh		\