RTC reset test was failing when board has been just powered and RTC reset test was executed for the first time (issue has been detected on CI). In such case RTC initialization takes more time than in futher rtc_init calls. This has impact on green-tea communication. Commands send by host immediately after init command (write, read) were not handled on the device side and no response to the host was provided.
To fix this problem test communication flow has been modified. Device sends ack to the host after RTC init is done. Host sends further RTC commands when ack from the device has been received.

Edit:
There are still some communication issues on the CI. Add ack from the device after each executed command. Increase test timeout and wait after reset.