tests: numfmt: add non-utf8 multi-byte test

* tests/numfmt/mb-non-utf8.sh: Test GB18030 delimiter search.
* tests/local.mk: Reference the new test, and move
the existing numfmt.pl test from tests/misc to tests/numfmt.

tests/local.mk

@@ -359,7 +359,8 @@ all_tests =					\
   tests/nproc/nproc-avail.sh			\
   tests/nproc/nproc-positive.sh			\
   tests/nproc/nproc-override.sh			\
-  tests/misc/numfmt.pl				\
+  tests/numfmt/numfmt.pl			\
+  tests/numfmt/mb-non-utf8.sh			\
   tests/misc/option-aliases.sh			\
   tests/od/od-N.sh				\
   tests/od/od-j.sh				\

tests/numfmt/mb-non-utf8.sh

@@ -0,0 +1,34 @@
+#!/bin/sh
+# Test handling of non-utf8 locales
+
+# Copyright (C) 2025 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/>.
+
+. "${srcdir=.}/tests/init.sh"; path_prepend_ ./src
+print_ver_ numfmt printf
+
+export LC_ALL=zh_CN.gb18030
+
+test "$(locale charmap 2>/dev/null | sed 's/gb/GB/')" = GB18030 ||
+  skip_ 'GB18030 charset support not detected'
+
+# Requires support for strchr(), mbschr(), and mbsstr() respectively.
+# Note 0xFF is invalid in GB18030, but we support all single byte delimiters.
+for delim in ',' ':' "$(env printf '\xa2\xe3')" "$(env printf '\xff')"; do
+  num_out=$(numfmt --from=si --field=2 -d "$delim" "1${delim}2K")
+  test "$num_out" = "1${delim}2000" || fail=1
+done
+
+Exit $fail