samples/drivers/adc: Few fixes for use with twister

Add some changes to adc sample to enable valid verdict generation
using twister:
-Replace 'single_line' keyword with correct 'one_line'
-Remove parenthesis from the sample output to enable regex verdict
-Add timeout to save time in execution

Additionally, add support for disco_l475_iot1.

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>