tests/drivers/aon_counter: make a ztest testcase

Reorganize the code tree so no forward declarations are needed
(basically, move main() to the bottom).

Retag as not build only so it can be run on Quark*

Convert to actually verify counter readouts are increasing /
decreasing over busy empty loops and the alarm is being called.

Change-Id: I746efe595b8d1ac4471dccc2e87f8b36f5cc8ebe
Signed-off-by: Inaky Perez-Gonzalez <inaky.perez-gonzalez@intel.com>