Merge changes from topic "cactus_message_loop"
* changes:
Cactus: tidy message loop and commands definitions
SPM: Helpers for error logging after FF-A calls
FF-A: Tidying direct messaging API definitions
SPM: Properly placing Cactus test files
SPM: Tidying FF-A Memory Sharing tests
TFTF: Use CACTUS_ECHO_CMD in direct message tests
Cactus: Refactor handling of commands
Cactus: Extend arguments in commands responses
FF-A: Check FF-A return is a valid direct response
FF-A: FFA_MSG_DIRECT_RESP call using 5 registers