This tests initializes all on-board buttons available, currently up to 4 in total. Each button is configured as an input with interrupt on button press or release depending on the configured TEST_FLANK.
When a button is pressed (and/or released) an interrupt fires which results in a message indicating which button was used. The test naturally qualifies for manual testing only. Nevertheless, it prints "[SUCCESS]" after all buttons were initialized successfully or "[FAILED] " on error, so it can be used for limited automated testing as well.