tests/thread/stress_aes.py: Reduce test time on PC targets.

This thread stress test is quite intensive and can run for a long time on
certain targets.  For example, builds with stackless enabled are slower
than non-stackless, and this test in stackless mode takes around 2 minutes
on the unix port of MicroPython with the existing parameters of
`n_thread=20` and `n_loop=5`.

It's not really necessary to test 20 threads at once, that's not really
going to test anything more than 10 at once.  So reduce the parameters to
keep the running time reasonable.

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

tests/thread/stress_aes.py

@@ -277,7 +277,7 @@ if __name__ == "__main__":
         n_thread = 2
         n_loop = 2
     else:
-        n_thread = 20
+        n_thread = 10
         n_loop = 5
     for i in range(n_thread):
         _thread.start_new_thread(thread_entry, (n_loop,))