test-casefolding.t: demonstrate a bug with HFS+ ignoring some codepoints

tests/test-casefolding.t

@@ -195,4 +195,17 b' case changes.'
   $ hg qrefresh a # issue 3271, qrefresh with file handled case wrong
   $ hg status # empty status means the qrefresh worked
 
+#if osx
+
+We assume anyone running the tests on a case-insensitive volume on OS
+X will be using HFS+. If that's not true, this test will fail.
+
+Bug: some codepoints are to be ignored on HFS+:
+
+  $ rm A
+  >>> open(u'a\u200c'.encode('utf-8'), 'w').write('unicode is fun')
+  $ hg status
+  M A
+  ? a\xe2\x80\x8c (esc)
+#endif
   $ cd ..