Add a crasher for the thread-unsafety of file objects.

Lib/test/crashers/file_threads.py

@@ -0,0 +1,8 @@
+# An example for http://bugs.python.org/issue815646
+
+import thread
+
+while 1:
+    f = open("/tmp/dupa", "w")
+    thread.start_new_thread(f.close, ())
+    f.close()