c32srtombs: Add tests. * tests/test-c32srtombs.c: New file, based on tests/test-wcsrtombs.c. * tests/test-c32srtombs-1.sh: New file, based on tests/test-wcsrtombs1.sh. * tests/test-c32srtombs-2.sh: New file, based on tests/test-wcsrtombs2.sh. * tests/test-c32srtombs-3.sh: New file, based on tests/test-wcsrtombs3.sh. * tests/test-c32srtombs-4.sh: New file, based on tests/test-wcsrtombs4.sh. * modules/c32srtombs-tests: New file, based on modules/wcsrtombs-tests.