Install stack trace handlers in unit tests

Summary: Sometimes, our tests fail because of normal `assert` call. It would be helpful to see stack trace in that case, too.

Test Plan: Added `assert(false)` and verified it prints out stack trace

Reviewers: haobo, dhruba, sdong, ljin, yhchiang

Reviewed By: haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D18291