tests/kernel/smp: Clean up "guess waiting" on SMP thread exit

The various tests would all do a "wait for threads to exit" step
before checking the results, but this was implemented with a simple
busy wait that turns out to need careful tuning (because there was
busy waiting in the threads).

Rather than try to synchronize this, white box the issue (it's a low
level SMP test, after all) by spinning on the thread states directly
watching for the kernel to flag them dead.  The downside here is that
if the process fails for some reason we'll get a hang and a timeout
reported from sanitycheck and not a synchronous ztest assertion.  But
in return, successful tests run much faster and I don't need to worry
about how to tune them for IPI latency on different platforms.

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