run-tests: call Threads constructor with keyword arguments

This is suggested in the python documentation
(on http://docs.python.org/2/library/threading.html#thread-objects)

tests/run-tests.py

@@ -1089,7 +1089,7 b' def scheduletests(options, tests):'
                 test = tests.pop(0)
                 if options.loop:
                     tests.append(test)
-                t = threading.Thread(None, job, args=(test, count))
+                t = threading.Thread(target=job, args=(test, count))
                 t.start()
                 running += 1
                 count += 1