Fix intermittent failure in multi-threading related tests

The tests related to multi-threading (in this file: [test_multithreaded.py](https://github.com/tuxmonk/pupdb/blob/master/tests/test_multithreaded.py)) fail intermittently. Speculated cause is thread(s) starvation, but need to dig deeper. Find out what's the cause and reduce/fix the intermittent failures.