[Bug libstdc++/67096] libstdc++ testsuite, codecvt: many UTF-8 tests illegal (testing bytes 5 and 6)

[gnugcc at marino dot st]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67096

--- Comment #4 from John Marino <gnugcc at marino dot st> ---
Created attachment 36332
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=36332&action=edit
codecvt/max_length/wchar/4.cc patch

codecvt/max_length/wchar/4.cc test thinks that 6 is the maximum byte size for
UTF-8 and fails if it detects lengths less than 6 bytes.

Of course, the true limit is 4 bytes, so the test needs to be changed from 6 to
4.

It is related to the tests this PR fixed (same problem, different form).