Added tests for covering initialization error handling cases like; NULL pointer, invalid GPIO count etc.
This changed implied removing the `relay_chn_create` from the Unity's `setUp` function and place it in each testcase.
Refs #1050, #1030.
- Replaced the buggy, oldschool, plain pointer based list approach with more robust FreeRTOS linked list implementation for the listener API. Fixes #1049.
- Added relevant test cases. Refs #1030.
The component allocates resources (timers, event loop) in relay_chn_create but never frees them. This is a resource leak.
Hence, a destroy function added to free the resources gracefully.
Fixes #1048.
