Summary: We should use mocked-out env for these tests to make it more realiable. Added benefit is that instead of actually sleeping for 3 seconds, we can instead pretend to sleep and just increase time counters.

Test Plan: for i in `seq 100`; do ./wal_manager_test --gtest_filter=WalManagerTest.WALArchivalTtl ;done

Reviewers: rven, meyering

Reviewed By: meyering

Subscribers: meyering, dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D36951