Summary:
Move reusable part of db_test.cc to util/db_test_util.h.
This makes it more possible to partition db_test.cc into
multiple smaller test files.
Also, fixed many old lint errors in db_test.
Test Plan: db_test
Reviewers: igor, anthony, IslamAbdelRahman, sdong, kradhakrishnan
Reviewed By: sdong, kradhakrishnan
Subscribers: dhruba, leveldb
Differential Revision: https://reviews.facebook.net/D41973
