tests/driver/adc: Add ADC accuracy test

These tests, that expects boards to be properly setup, allow one to
measure ADC value and compare to expected values.

Two tests are added. For one, a reference voltage is expected on the ADC
- and this value can be informed to the test via devicetree. The other
uses DAC to generate a value that is then read from ADC. This assumes
DAC is accurate. Naturally, one could make this a DAC test if ADC is
assumed to be accurate.

As the board setup involve connecting physical pins, they are behind
twister fixtures.

Signed-off-by: Yusuf Ahmed <muhammed.ahmed@intel.com>
Signed-off-by: Ederson de Souza <ederson.desouza@intel.com>