parse-datetime-tests: port to Alpine Linux 3.12.1 * tests/test-parse-datetime.c: Include errno.h for errno, and unistd.h for _SC_TZNAME_MAX and sysconf. (main): In the outlandishly-long time zone abbreviation test, do not exceed TZNAME_MAX as this has undefined behavior, and on Alpine Linux 3.12.1 it makes the test fail.