tests/thread: Make stress_create.py test run on esp32.

The esp32 port needs to be idle for finished threads and their resources to
be freed up.

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

tests/thread/stress_create.py

@@ -1,9 +1,13 @@
 # stress test for creating many threads
 
 try:
-    import utime as time
+    import utime
+
+    sleep_ms = utime.sleep_ms
 except ImportError:
     import time
+
+    sleep_ms = lambda t: time.sleep(t / 1000)
 import _thread
 
 
@@ -16,9 +20,11 @@ while thread_num < 500:
     try:
         _thread.start_new_thread(thread_entry, (thread_num,))
         thread_num += 1
-    except MemoryError:
+    except (MemoryError, OSError) as er:
-        pass
+        # Cannot create a new thead at this stage, yield for a bit to
+        # let existing threads run to completion and free up resources.
+        sleep_ms(50)
 
 # wait for the last threads to terminate
-time.sleep(1)
+sleep_ms(500)
 print("done")