nstrftime: Add tests of all directives, also in non-trivial locales.
* tests/test-nstrftime.h (language_t): New type.
(locales_test): New function.
* tests/test-nstrftime.c: Include <locale.h>, <stdlib.h>.
(main): Invoke setlocale and locales_test.
* tests/test-nstrftime-1.sh: New file.
* tests/test-nstrftime-2.sh: New file, based on tests/test-strtod1.sh.
* modules/nstrftime-tests (Files): Add them. Add m4/locale-fr.m4,
m4/codeset.m4, m4/musl.m4.
(configure.ac): Invoke gt_LOCALE_FR, gt_LOCALE_FR_UTF8, gl_MUSL_LIBC.
(Makefile.am): Link test-nstrftime with $(SETLOCALE_LIB). Test
test-nstrftime-*.sh instead of test-nstrftime.