tests: sys_sem: add k_thread_join() calls

There is a race condition between the child threads
exiting, and the child threads getting re-used in the
next scenario. This reproduces more often on SMP systems.

Close the race by joining on the child threads before
exiting any test scenario.

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>

Author: Andrew Boie

tests/kernel/mem_protect/sys_sem/src/main.c

@@ -276,6 +276,8 @@ void test_sem_take_timeout(void)
 	ret_value = sys_sem_take(&simple_sem, SEM_TIMEOUT);
 	zassert_true(ret_value == 0,
 		     "sys_sem_take failed when its shouldn't have");
+
+	k_thread_join(&sem_tid, K_FOREVER);
 }
 
 /**
@@ -300,6 +302,8 @@ void test_sem_take_timeout_forever(void)
 	ret_value = sys_sem_take(&simple_sem, K_FOREVER);
 	zassert_true(ret_value == 0,
 		     "sys_sem_take failed when its shouldn't have");
+
+	k_thread_join(&sem_tid, K_FOREVER);
 }
 
 /**
@@ -318,6 +322,8 @@ void test_sem_take_timeout_isr(void)
 	ret_value = sys_sem_take(&simple_sem, SEM_TIMEOUT);
 	zassert_true(ret_value == 0,
 		     "sys_sem_take failed when its shouldn't have");
+
+	k_thread_join(&sem_tid, K_FOREVER);
 }
 
 /**
@@ -418,6 +424,9 @@ void test_sem_take_multiple(void)
 	zassert_true(signal_count == 1U,
 		     "low priority thread didn't get executed");
 
+	k_thread_join(&sem_tid, K_FOREVER);
+	k_thread_join(&sem_tid_1, K_FOREVER);
+	k_thread_join(&sem_tid_2, K_FOREVER);
 }
 
 /**
@@ -546,6 +555,10 @@ void test_sem_multiple_threads_wait(void)
 			     signal_count);
 
 		repeat_count++;
+
+		for (int i = 0; i < TOTAL_THREADS_WAITING; i++) {
+			k_thread_join(&multiple_tid[i], K_FOREVER);
+		}
 	} while (repeat_count < 2);
 }