tests/kernel/timer_api: Synchronize racy subtest

The test_timer_periodicity test is racy and subject to initial state
bugs.  The operation of that test is to:

1. Start a timer with a known period
2. Take the current time with k_uptime_get()
3. Wait for the timer to fire with k_timer_status_sync()
4. Check that the current time minus start time is the period

But that's wrong, because a tick expiring between any of the first
three steps is going to skew the math (i.e. the timer will have
started on a different tick than the "start time").

And taking an interrupt lock around the process can't fix the issue,
because in the tickless world we live in k_uptime_get() is actually a
realtime quanity based on a hardware counter and doesn't rely on
interrupt delivery.

Instead, use another timer object to synchronize the test start to a
driver tick, ensuring that even if the race is unfixable the initial
conditions are always correct.

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