tests: mem_protect/syscall: join stress threads after abort

Add a loop to join the stress threads after calling abort.
This is to make sure all stress threads have already stopped
before moving on. Simply for test hygiene.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>

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

@@ -407,6 +407,10 @@ ZTEST(syscalls_extended, test_syscall_switch_stress)
 		k_thread_abort(&stress_threads[i]);
 	}
 
+	for (i = 0; i < NR_THREADS; i++) {
+		k_thread_join(&stress_threads[i], K_FOREVER);
+	}
+
 	printk("\n");
 }