tests/kernel/timer/timer_api: Add tests for the new conversion routines

The new conversion API has a ton of generated utilities.  Test it via
enumerating each one of them and throwing a selection of both
hand-picked and random numbers at it.  Works by using slightly
different math to compute the expected result and assuming that we
don't have symmetric bugs in both.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>