tests: drivers: counter: add rtc_ds3231_api

Cloned from counter_basic_api with modifications based on DS3231
limitations:
* Only one device tested per board;
* Counter cannot be stopped or started;
* Alarms are serviced by worker thread, so are not invoked from ISR
  and require that test thread yield to allow processing (no
  k_busy_wait());
* Multiple Alarms test is disabled as documented in test.

Additional tests were added for DS3231-specific API.

Signed-off-by: Peter A. Bigot <pab@pabigot.com>