mbrtowc: Add more tests for native Windows platforms.
* tests/test-mbrtowc-w32-1.sh: New file.
* tests/test-mbrtowc-w32-2.sh: New file.
* tests/test-mbrtowc-w32-3.sh: New file.
* tests/test-mbrtowc-w32-4.sh: New file.
* tests/test-mbrtowc-w32-5.sh: New file.
* tests/test-mbrtowc-w32.c: New file.
* modules/mbrtowc-tests (Files): Add them.
(Makefile.am): Arrange to run these tests.
* tests/test-mbrtowc-w32-6.sh: New file, currently unused.
* tests/test-mbrtowc-w32-7.sh: New file, currently unused.