add an env var ROCKSDB_TESTS_FROM to control where to start from a list of tests

Summary:
Sometimes, I got a test failure. After fixing that, I want to resume
db_test from that test. ROCKSDB_TESTS_FROM is for this purpose.

Test Plan: as title

Reviewers: yhchiang, rven, igor, sdong

Reviewed By: sdong

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D27807