Issue #12016: Multibyte CJK decoders now resynchronize faster

They only ignore the first byte of an invalid byte sequence.

For example, b'\xff\n'.decode('gb2312', 'replace') gives '\ufffd\n' instead of
'\ufffd'.
diff --git a/Lib/test/test_codecmaps_tw.py b/Lib/test/test_codecmaps_tw.py
index 6db5091..412b9de 100644
--- a/Lib/test/test_codecmaps_tw.py
+++ b/Lib/test/test_codecmaps_tw.py
@@ -23,6 +23,9 @@
         (b'\xa2\xcc', '\u5341'),
         (b'\xa2\xce', '\u5345'),
     ]
+    codectests = (
+        (b"\xFFxy", "replace",  "\ufffdxy"),
+    )
 
 def test_main():
     support.run_unittest(__name__)