tests: add ending logic for the preempt test

Old tests/kernel/sched/preempt cannot be run repeatedly.
If you register the test_preempt() method twice, the
second run will fail. It is because the participating
threads don't exit properly when the main thread exits.

The new ztest fx implies that a unit test should be able
to run repeatedly. This commit enables that for the preempt
test by adding explicit epilogue.

Signed-off-by: Ming Shao <ming.shao@intel.com>

tests/kernel/sched/preempt/src/main.c

@@ -346,6 +346,21 @@ void test_preempt(void)
 	 * test is done
 	 */
 	k_sem_take(&main_sem, K_FOREVER);
+
+	/* unit test clean up */
+
+	/* k_thread_abort() also works here.
+	 * But join should be more graceful.
+	 */
+	k_thread_join(&manager_thread, K_FOREVER);
+
+	/* worker threads have to be aborted.
+	 * It is difficult to make them stop gracefully.
+	 */
+	for (int i = 0; i < NUM_THREADS; i++) {
+		k_thread_abort(&worker_threads[i]);
+	}
+
 }
 
 void test_main(void)