[committed] libstdc++: Use IANA name for ISO-8859-1 in format tests.

[Tomasz Kamiński]

Use IANA name for ISO-8859-1 as "-fexec-charset=" and add dg-require-iconv,
to make sure it is supported.

libstdc++-v3/ChangeLog:

        * testsuite/std/format/debug_nonunicode.cc: Pass ISO-8859-1 as
        exec-charset and make sure that it is supported.
        * testsuite/std/format/fill_nonunicode.cc: Likewise.
---
Updating existing test, per Jakub's suggestion from:
https://gcc.gnu.org/pipermail/libstdc++/2026-May/066497.html

Tested on x86_64-linux. Pushed to trunk.

 libstdc++-v3/testsuite/std/format/debug_nonunicode.cc | 3 ++-
 libstdc++-v3/testsuite/std/format/fill_nonunicode.cc  | 3 ++-
 2 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/libstdc++-v3/testsuite/std/format/debug_nonunicode.cc 
b/libstdc++-v3/testsuite/std/format/debug_nonunicode.cc
index 2ac7e757ea8..de70f55f14a 100644
--- a/libstdc++-v3/testsuite/std/format/debug_nonunicode.cc
+++ b/libstdc++-v3/testsuite/std/format/debug_nonunicode.cc
@@ -1,4 +1,5 @@
-// { dg-options "-fexec-charset=ISO8859-1" }
+// { dg-options "-fexec-charset=ISO-8859-1" }
+// { dg-require-iconv "ISO-8859-1" }
 // { dg-do run { target c++23 } }
 // { dg-add-options no_pch }
 
diff --git a/libstdc++-v3/testsuite/std/format/fill_nonunicode.cc 
b/libstdc++-v3/testsuite/std/format/fill_nonunicode.cc
index 1370cf57ccd..16d858c93a8 100644
--- a/libstdc++-v3/testsuite/std/format/fill_nonunicode.cc
+++ b/libstdc++-v3/testsuite/std/format/fill_nonunicode.cc
@@ -1,4 +1,5 @@
-// { dg-options "-fexec-charset=ISO8859-1" }
+// { dg-options "-fexec-charset=ISO-8859-1" }
+// { dg-require-iconv "ISO-8859-1" }
 // { dg-do run { target c++20 } }
 // { dg-add-options no_pch }
 
-- 
2.54.0