tests/kernel: Simplify timer spinning

There is actually nothing wrong with this test code idiom.  But it's
tickling a qemu emulator bug with the hpet driver and x86_64[1].  The
rapidly spinning calls to k_uptime_get_32() need to disable
interrupts, read timer hardware state and enable them.  Something goes
wrong in qemu with this process and the timer interrupt gets lost.
The counter blows right past the comparator without delivering its
interrupt, and thus the interrupt won't be delivered until the counter
is next reset in idle after exit from the busy loop, which is
obviously too late to interrupt the timeslicing thread.

Just replace the loops with a single call to k_busy_wait().  The
resulting code ends up being much simpler anyway.  An added bonus is
that we can remove the special case handling for native_posix (which
was an entirely unrelated thing, but with a similar symptom).

[1] But oddly not the same emulated hardware running with the same
driver under the same qemu binary when used with a 32 bit kernel.

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