tests: introduce tests for LED drivers

This patch introduces some tests for the LED drivers. The following
functions of the LED API are tested:

- led_get_info
- led_on
- led_off
- led_set_color
- led_set_brightness

Note that the led-controller-0 alias must be defined in the DTS of the
board used to run this tests.

Signed-off-by: Simon Guinot <simon.guinot@seagate.com>