tests: drivers: adc_dma: fix buffer state not being verified

Previously the contents of buffers after an ADC DMA read was
simply printed, but not verified with an zassert that the
value was updated.

This commit updates it to be similar to the adc_api test that
fills the buffer initially with a known value, which is then
used to ensure the ADC DMA functioned successfully.

Signed-off-by: Hein Wessels <heinwessels93@gmail.com>