This patch adds tests for the mbrtoc8 and c8rtomb functions adopted for 
C++20 via WG21 P0482R6 [1] and for C2X via WG14 N2653 [2], and for the 
char8_t typedef adopted for C2X via WG14 N2653 [2].

The tests for mbrtoc8 and c8rtomb specifically exercise conversion 
from/to Big5-HKSCS because of special cases that arise with that 
encoding. Big5-HKSCS defines some double byte sequences that convert to 
more than one Unicode code point. In order to test this, the locale 
dependencies for running tests under wcsmbs is expanded to include 
zh_HK.BIG5-HKSCS.

Tested on Linux x86_64.

Tom.

[1]: WG21 P0482R6
      "char8_t: A type for UTF-8 characters and strings (Revision 6)"
      https://wg21.link/p0482r6

[2]: WG14 N2653
      "char8_t: A type for UTF-8 characters and strings (Revision 1)"
      http://www.open-std.org/jtc1/sc22/wg14/www/docs/n2653.htm