tests/thread/stress_aes.py: Fix logic waiting for finished threads.

Because the main thread executes `thread_entry()` it means there's an
additional one added to `count`, so the test must wait for the count to
reach `n_thread + 1`.

Signed-off-by: Damien George <damien@micropython.org>

tests/thread/stress_aes.py

@@ -282,6 +282,6 @@ if __name__ == "__main__":
     for i in range(n_thread):
         _thread.start_new_thread(thread_entry, (n_loop,))
     thread_entry(n_loop)
-    while count.value < n_thread:
+    while count.value < n_thread + 1:
         time.sleep(1)
     print("done")