tests/drivers/adc: move to ztest to actually test

This TC is only exercising the API, as we don't have a feedback loop
mechanism to verify whichever values are fed to the ADC.

Fixed the loop to complete after 10 runs; on each run, print the
values and actually report the difference between them. With no inputs
connected (aka: floating), they should be reporting noise relatively
close to the previous reading, so we might want to use this delta as a
testing pattern (assert if the delta is higher than some value, but
I've seen variations as high as 40M units). For now, the test is just
happy with being able to read them.

The buffer has been re-typed to uint32_t so we can iterate over it
without casting tricks -- it requires then only a single cast when
initializing sample.buffer (which shall be a void* anyway).

Duplicated the buffer, so we can flip/flop between two buffers to
compare against the entries read in the previous run.

v4: fixed missed warnings

Change-Id: If6b48b92231007202d74f5c042f6d0cf3fdcb60a
Signed-off-by: Inaky Perez-Gonzalez <inaky.perez-gonzalez@intel.com>
(cherry picked from commit 9e1df6f2)