time_r: Add tests. * lib/time_r.c: Add comment. * tests/test-gmtime_r.c: New file. * tests/test-gmtime_r-mt.c: New file, based on tests/test-nl_langinfo-mt.c. * tests/test-localtime_r.c: New file. * tests/test-localtime_r-mt.c: New file, based on tests/test-nl_langinfo-mt.c. * modules/time_r-tests: New file.