tests: Add tests for _thread module.

Includes functionality and stress tests.

tests/thread/thread_lock2.py

@@ -0,0 +1,24 @@
+# test _thread lock objects with multiple threads
+#
+# MIT license; Copyright (c) 2016 Damien P. George on behalf of Pycom Ltd
+
+try:
+    import utime as time
+except ImportError:
+    import time
+import _thread
+
+lock = _thread.allocate_lock()
+
+def thread_entry():
+    lock.acquire()
+    print('have it')
+    lock.release()
+
+# spawn the threads
+for i in range(4):
+    _thread.start_new_thread(thread_entry, ())
+
+# wait for threads to finish
+time.sleep(0.2)
+print('done')