Summary:
Added make targets parallel_test and parallel_dbtest to run
tests in parallel. Each test is run 32 times in parallel. There is a
timeout to catch hangs. The test continues after a failure and reports
non-zero status on failure

Test Plan: Run the two make targets

Reviewers: anthony, yhchiang, IslamAbdelRahman, kradhakrishnan, sdong

Reviewed By: sdong

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D53079