Summary:
(a) use STRESS_LIBRARY for db_stress and make sure
STRESS_LIBRARY has other stress test dependencies (as in buck build)

(b) fix rpath option to be accepted on MacOS. It still doesn't fully work
for me e.g. to run a LIB_MODE=shared unit test binary from another
directory, as it does on Linux, but the option is now accepted, and running
unit tests from current directory works for me.

Also adding LIB_MODE=shared to Travis. (Later TBD where best to fit in
in CircleCI.)
Pull Request resolved: https://github.com/facebook/rocksdb/pull/7066

Test Plan: manual

Reviewed By: cheng-chang

Differential Revision: D22364068

Pulled By: pdillinger

fbshipit-source-id: 6fa98a222f89f808ee786474de1100d92c1adec3