Fix GetProperty() test

Summary:
GetProperty test is flakey.
Before this diff: P8635927
After: P8635945

We need to make sure the thread is done before we destruct sleeping tasks. Otherwise, bad things happen.

Test Plan: See summary

Reviewers: ljin, sdong, haobo, dhruba

Reviewed By: ljin

CC: leveldb

Differential Revision: https://reviews.facebook.net/D17595