[SystemZ][z/OS][libcxx]: fix libcxx test cases related to codecvt class UTF16/32

This PR is to fix a few UTF16 and UTF32 related test cases that are testing member functions for https://en.cppreference.com/w/cpp/locale/codecvt class , functions are converting from UTF16, UTF32 to UTF8 or vise visa. Test cases need to explicitly specify it is UNICODE character for UTF16/32 type in order to be valid tests to match type in documentation. it assumes it will be ASCII or UTF8 type for 1 byte character ( value range from 1 to 127 ), which is not true on z/OS in EBCDIC mode. For information related to UTF16/32 , please see https://naveenr.net/unicode-character-set-and-utf-8-utf-16-utf-32-encoding/ , and EBCDIC/ASCII character value can be found in http://www.simotime.com/asc2ebc1.htm

Differential Revision: https://reviews.llvm.org/D106151