tests/thread: Add stress-test for creating many threads.

tests/thread/stress_create.py

+# stress test for creating many threads
+
+try:
+    import utime as time
+except ImportError:
+    import time
+import _thread
+
+def thread_entry(n):
+    pass
+
+thread_num = 0
+while thread_num < 500:
+    try:
+        _thread.start_new_thread(thread_entry, (thread_num,))
+        thread_num += 1
+    except MemoryError:
+        pass
+
+# wait for the last threads to terminate
+time.sleep(1)
+print('done')