This test will blink all available on-board LEDs, one after each other in an endless loop. Each LED will light up once long, and twice short, where the long interval is roughly 4 times as long as the short interval. The length of the interval is not specified and differs for each platform.
Running this test shows if all the direct access macros for all on-board LEDs are working correctly (xx_ON, xx_OFF, xx_TOGGLE). This test is intentionally not using any timers, so it can also be used early on in the porting process, where timers might not yet be available.