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

(cherry picked from commit c411c431)