testsuite: Encoding test witnesses recent iconv bug is fragile

A regression in the new iconv() distributed with XCode 15 and MacOS
Sonoma causes the test 'encoding004' to fail in the CP936 roundrip.

We mark this test as fragile until this is fixed upstream (rather than
broken, since previous versions of iconv pass the test)

See #24161

libraries/base/tests/IO/all.T

@@ -136,7 +136,11 @@ test('encoding004', [extra_files(['encoded-data/']), js_broken(22374),
 # and
 # https://gitlab.haskell.org/ghc/wasi-libc/-/blob/main/libc-top-half/musl/src/locale/codepages.h
 # for locales supported by wasi-libc's iconv implementation
-when(arch('wasm32'), skip)], compile_and_run, [''])
+when(arch('wasm32'), skip),
+# MacOS Sonoma iconv() has a regression that causes this test to fail on the
+# CP936 roundtrip. See the ticket for related issues in other projects.
+when(opsys('darwin'), fragile(24161))
+], compile_and_run, [''])
 test('encoding005', normal, compile_and_run, [''])
 
 test('environment001', [], makefile_test, ['environment001-test'])